Add support for .Net Core scripting

[Krzysztof-Cieslak]

This is work by me @baronfel to get back good scripting experience in FSAC.

We need to support following case:

• .Net Framework FSAC - support fsi.exe and use net462 references
• .Net Core FSAC - support fsi.exe (net462 references) and dotnet fsi (netcoreapp3.0 references). User can choose which one they want to target with UseSdkScripts settings.

Currently some logic around resolving correct set of references is in this PR rather than FCS - dotnet/fsharp#7701

Discussion about FSI support in FSAC - #465

[baronfel]

With the latest changes we should be ready to go. To summarize:

• if the user opts-in to netcore scripts, we:
  • assume a default install location for the sdk (could be easily configured)
  • detect the most up-to-date SDK at least version 3.0.100
  • detect the most up-to-date runtime at least version 3.0.0
  • assume netcoreapp3.0 TFM
  • yield assembly references from the FSharp SDK always
  • yield assembly references from the matching pack directory for that runtime version + TFM

This should be reasonably future-proof while ensuring a minimum standard of support.

I also threaded through the netfx/netcore flag into the background processing/parsing of files, which required a data contract change and looks invasive, but otherwise was pretty mechanical.

This works great for me on OSX, should work similarly on any windows/unix machines so long as the documentation about default install locations is correct. Next steps would be to review/merge, then in Ionide use the user's UseSdkScripts setting to decide if we launch fsi/fsharpi like we do currently, or dotnet fsi.