Add support for abi stable module API (N-API) as "Experimental feature".
The goal of this API is to provide a stable Node API for native
module developers. N-API aims to provide ABI compatibility guarantees
across different Node versions and also across different
Node VMs - allowing N-API enabled native modules to just work
across different versions and flavors of Node.js without recompilation.

A more detailed introduction is provided in:
https:/nodejs/node-eps/blob/master/005-ABI-Stable-Module-API.md
and https:/nodejs/abi-stable-node/blob/doc/VM%20Summit.pdf.

The feature, during its experimental state, will be guarded by a runtime
flag "--napi-modules". Only when this flag is added to the command line
will N-API modules along with regular non N-API modules be supported.

The API is defined by the methods in "src/node_api.h",
"src/node_api_types.h" and "src/node_api_async.h". This is the best
starting point to review the API surface. More documentation will follow.

In addition to the implementation of the API using V8, which is included
in this PR, the API has also been validated against chakracore and that
port is available in
https:/nodejs/abi-stable-node/tree/api-prototype-chakracore-8.x.

The current plan is to provide N-API support in versions 8.X and 6.X
directly. For older versions, such as 4.X or pre N-API versions of 6.X,
we plan to create an external npm module to provide a migration path
that will allow modules targeting older Node.js versions to use the API,
albeit without getting the advantage of not having to recompile.

In addition, we also plan an external npm package with C++ sugar to
simplify the use of the API. The sugar will be in-line only and will
only use the exported N-API methods but is not part of the N-API
itself. The current version is in:
https:/nodejs/node-api.

This PR is a result of work in the abi-stable-node repo:
https:/nodejs/abi-stable-node/tree/doc,
with this PR being the cumulative work on the api-prototype-8.x
branch with the following contributors in alphabetical order:

 Arunesh Chandra (@aruneshchandra)
 Gabriel Schulhof (@gabrielschulhof)
 Hitesh Kanwathirtha (@digitalinfinity)
 Ian Halliday (@ianwjhalliday)
 Jason Ginchereau (@jasongin)
 Michael Dawson (@mhdawson)
 Sampson Gao (@sampsongao)
 Taylor Woll (@boingoing)

The snprintf API is not available with older versions of MSVC. Found by a CI run.

This updates the documentation, the error message upon module load
failure, the command line option parsing of the flag, and the way
the N-API addon tests pass the flag to node.

Re nodejs#11975 (comment)
Re nodejs#11975 (comment)
Fixes nodejs/abi-stable-node#184
Closes nodejs/abi-stable-node#186

Some tests need to stay C++ because they define classes.

Fixes nodejs/abi-stable-node#183
Closes nodejs/abi-stable-node#190

* Convert all locals and parameter names to snake_case

Standardize some parameter names across the N-API surface such as ```napi_env env``` and ```size_t length```. Make all the parameter names in the header match the cc file and expand some parameter names from single characters into meaningful words.

Also addresses a couple of minor PR feedback items:
 - Rename ```napi_reference_addref``` and ```napi_reference_release``` to ```napi_reference_ref``` and ```napi_reference_unref``` (respectively)
 - Rename ```Reference::AddRef``` and ```Reference::Release``` to ```Reference::Ref``` and ```Reference::Unref``` (respectively)
 - Remove ```napi_create_boolean```, ```napi_get_true```, and ```napi_get_false``` and replace them with ```napi_get_boolean```
 - Rename ```napi_get_type_of_value``` to ```napi_typeof```
 - Add a ```result_data``` out parameter to ```napi_create_buffer_copy``` which returns the data pointer of the new buffer
 - Change ```napi_get_value_string_utf8``` and ```napi_get_value_string_utf16``` to accept a null output buffer and return the length of the source string in that case via the ```result``` parameter
 - Remove ```napi_get_value_string_utf8_length``` and ```napi_get_value_string_utf16_length``` due to the above
 - Change ```Reference``` ctor and dtor to be protected and added public ```static Reference* Reference::New``` and ```static void Reference::Delete``` methods to make it more clear how the ```Reference``` objects are allocated and cleaned-up
 - Add a type check to ``napi_get_array_length``` which returns an error if the argument is not an array object
 - Change ```napi_create_symbol``` to take a ```napi_value``` instead of a ```const char*``` for the symbol description string. The API now throws if description is not a string but NULL is allowed.

* Remove UTF-8 BOM characters and other non-ANSI whitespace

* Change parameter names for create string APIs and address other feedback

* Fixes for unit tests

* Fix lint errors found via CI

 - Simplify conversion to/from napi_value
 - Better names for CallbackWrapper template params
 - Fatal error when trying to set setter return value
 - Use node::arraysize
 - Replace v8::Handle with v8::Local
 - Align the star after v8::Isolate

Re nodejs#11975 (comment)
Fixes nodejs/abi-stable-node#182
Closes nodejs/abi-stable-node#185

 - Check the type before casting to string when creating error objects.
 - Return an error when napi_typeof does not find any valid type.

 - Delete the node_api_*internal.h headers
 - Don't crash when casting numbers when
   V8_ENABLE_CHECKS is defined
 - Return 0 when getting NaN value as an integer
 - Always set callback data internal field to prevent
   crashing when V8_ENABLE_CHECKS is defined
 - Style: replace `if (ptr)` with `if (ptr != nullptr)`
 - Fix extern "C" in node_api_async.h
 - Minor doc changes

 - Consistent documentation for --napi-modules
 - Add underscore to napi_get_property_names
 - Avoid using V8 APIs marked as pending deprecation
 - Avoid unnecessary copying of args arrays
 - Other miscellaneous cleanup

 - Add an env parameter to napi_finalize
 - Remove unnecessary reinterpret_cast<> calls
 - Remove redundant Maybe.IsNothing() checks
 - Make function call result parameters optional
 - Wrap internal field pointer values in External
 - Fix broken callback tests and add error-checking
 - Other misc test cleanup

* Refcount for v8impl::Reference should be unsigned

Add ```Reference::RefCount()``` and check the refcount value before trying to call ```Reference::Unref()``` to avoid underflowing the refcount. This allows us to continue returning an error from ```napi_reference_unref``` if it is called with a reference that already has refcount set to zero.

* Add check for _refcount == 0 in Reference::Unref to avoid an underflow

- Check that an internal field value is actually an External before casting.
   This prevents a crash if the wrong object is passed to napi_unwrap.
 - Wrap some macro parameters in parentheses.

These APIs will be redesigned and added into the master branch after our main PR lands there.

See nodejs/abi-stable-node#204 to see progress on the redesigned APIs.

This makes it necessary to pass napi_env to napi_get_last_error_info()
thereby ensuring that errors can be tied to the napi_env in the future.

Re nodejs/abi-stable-node#198

* Add the _array suffix to the TypedArray type enum

Also remove a stale comment