feat(language server): add csharp-ls as a new option

[rbstp]

Adds csharp-ls as a third language server option alongside OmniSharp and Roslyn. csharp-ls is an F#-based C# language server distributed as a dotnet tool NuGet package; it is unrelated to the deprecated SofusA/csharp-language-server wrapper referenced by #11 and #57. The naming overlap is unfortunate: the upstream project markets itself as csharp-ls precisely to reduce that confusion.

The use case is different from Roslyn. csharp-ls is a leaner alternative: cold start is faster, memory footprint is lower, and it stays responsive on small projects or quick edits where spinning up a full Roslyn workspace is overkill. Roslyn remains the right choice for the deepest Microsoft tooling; csharp-ls is for the case when you want something snappy.

To use it:

{
  "languages": {
    "CSharp": {
      "language_servers": ["csharp-ls", "!omnisharp", "!roslyn"]
    }
  }
}

It partially addresses #62, it is running better on my own M3 mac for smaller projects.

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