[vcpkg] Add a schema file for vcpkg.json

[edhebi]

This is related to #12175 .

This is absolutely not as tested as it should be, but I'm not sure how to go about it. I gess some set of known valid and known invalid manifest files would be required.

I choose the path an filename fairly arbitrarily, feel free to move it around.

[ghost]

All CLA requirements met.

[edhebi]

Also, if that path ends up being the final one, an id field should be added with and url providing the schema, either the github raw one, or something else:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$id": "https://raw.githubusercontent.com/microsoft/vcpkg/master/scripts/vcpkg.schema.json",
  ...
}

[strega-nil]

I think that this is really good, the only thing I see is that it would be good to make the definitions match the spec: platformExpression -> platform-expression?

[edhebi]

I wondered wether too do that or use the conventional json casing. I'll change the case when I have some time

[edhebi]

done, all definitions are now with-dashes. I kept description as description-field to avoid mixing it up with the description keyword

[ras0219]

@edhebi did you intend to mark this as ready to merge instead of a draft?

[edhebi]

@ras0219 oops, my bad. I'll mark it as ready to merge

[strega-nil]

Thanks so much @edhebi!