add F# support for the WriteCodeFragment task

[brettfo]

Enables the <WriteCodeFragment /> task for F#.

Since F# doesn't have CodeDOM support, both the CodeDOM and CoreCLR branches end up calling the same function to do the generation. This should enable things like assembly name/version attributes to be generated.

Due to some oddities in how F# code must be structured, there are two things to point out in the generated code:

1. Files must (not really, but it's complicated) start with a namespace declaration. This value doesn't matter and won't end up in the compiled assembly, so the value of FSharp was selected.
2. F# files that contain no types/modules must have some executable code and the F# way of doing this is to add a do() expression at the bottom of the file.

Otherwise the code generated for CoreCLR C# was used as the model and a VB-specific test was copied.

Fixes #2584.

[AndyGerlicher]

This seems like it should be in the F# SDK? Ideally even C# / VB wouldn't be here either, but I'd at least rather not expand when possible. Any reason it couldn't go there?

[brettfo]

No reason this couldn't go in the F# SDK; I was just mimicking what C#/VB does. I'd be happy to have this PR closed and put it there if that's what you'd prefer.

[brettfo]

Closing in favor of dotnet/fsharp#3702