C# Support: Add treesitter and OmniSharp LSP support (#6908)

This PR adds the C# tree-sitter grammar. It also adds OmniSharp-Roslyn
for LSP support.

Resolves issue
[#5299](https://github.com/zed-industries/zed/issues/5299)

Release Notes:

- Added C# support

## VSCode
<img width="984" alt="vscode"
src="https://github.com/zed-industries/zed/assets/6967829/1f6b4cb7-4e00-4d61-8e58-2867dc5c8ecf">

## Zed
<img width="1722" alt="zed"
src="https://github.com/zed-industries/zed/assets/6967829/88436c78-93de-4e26-be15-b0dea6590c55">

docs/src/languages/csharp.md

@@ -0,0 +1,4 @@
+# C#
+
+- Tree Sitter: [tree-sitter-c-sharp](https://github.com/tree-sitter/tree-sitter-c-sharp)
+- Language Server: [OmniSharp](https://github.com/OmniSharp/omnisharp-roslyn)