Ported ILMergeHelper.fs to FAKE 5 as Fake.DotNet.ILMerge

[SteveGilham]

Description

Take the old ILMerge task and update it

• The ILMerge code is bound to the .net framework, so won't even work under mono on Windows, so the tests are restricted as for FxCop
• just as with FxCop, I've been using this code in my own build process via #load, so has some operational test validation as well

TODO

I believe these all to be done

• [] New (API-)documentation for new features exist (Note: API-docs are enough, additional docs are in help/markdown)
• [] unit or integration test exists (or short reasoning why it doesn't make sense)
• [] boy scout rule: "leave the code behind in a better state than you found it" (fix warnings, obsolete members or code-style in the places you worked in)
• [] (if new module) the module has been linked from the "Modules" menu, edit help/templates/template.cshtml, linking to the API-reference is fine.
• [] (if new module) the module is in the correct namespace
• [] (if new module) the module is added to Fake.sln (dotnet sln Fake.sln add src/app/Fake.*/Fake.*.fsproj)
• [] Fake 5 API guideline is honored

[matthid]

Honestly, if every PR would look like this maintaining would be much less work ;)
Thanks!

I added just one minor discussion point about the behavior on mono, we can leave it as-is but I feel like users might not realize why stuff is red.

[matthid]

Or we throw an exception in order to notify the user
`raise <| NotSupportedException("ILMerge is currently not supported on mono")

[matthid]

I guess I'd prefer throwing exception (as you can disable "Targets" via conditional targets already), but I can add that myself if I don't forget.
Thanks!