Support .rsp files for Scripting Project System bootstrapping

[filipw]

Spun off from dotnet/vscode-csharp#1856

It would be useful for people not using CSI or dotnet-script - especially those using custom CSX hosts - to be able to predefine using statements and assembly references via .rsp files. Those are standard files already used by the compiler itself and look like this: https://github.com/dotnet/roslyn/blob/version-2.4.0/src/Interactive/csi/csi.rsp

In our case, we'd pick up the references used there and use them as globally imported namespaces / references when constructing CSX project system. Configuration proposal - inside omnisharp.json, under script node, as rspPath property. For example:

Since omnisharp.json can be global, you could even - in one sweep - configure your entire DEV box if you wanted.

[KarloX2]

This is a great idea!

Ideally, referencing assemblies with /r would optionally allow the usage of environment variables, maybe like so:

/r:C:\Some\Specific\Folder\MyLib.dll
/r:%MY_ENV_VAR%\Folder\MyOtherLib.dll

[KarloX2]

cc me: @KarloX2

[filipw]

OK I've made some good progress on this - will PR this soon.

[KarloX2]

Very good! I'm really curious to see it. Can you please post here once the update is available? Thanks!