Support Solution filter (.slnf)

[JoeRobich]

Based on #1952 with changes for a passing CI.

[filipw]

LGTM. you will need to tweak the auto start in the extension too https://github.com/OmniSharp/omnisharp-vscode/blob/3117b4e9e053b4928ec979f360f93b39ab697c11/src/omnisharp/server.ts#L505

[JoeRobich]

you will need to tweak the auto start in the extension too

Yeah, I think the changes in dotnet/vscode-csharp#4076 should do it.

[AffluentOwl]

Any links to documentation on how to use this / what the intended workflow is? What if my project has hundreds of csproj files, is supposed to have no sln files, but a few spurious ones popped up that I want to filter out from omnisharp?

I'd like to avoid having to add and manage a sln file or slnf file for hundreds of projects I don't own, and instead just filter out all the bad sln files which are corrupting omnisharp.