languages/csharp: init

[diniamo]

Cleans up some stuff and adds a csharp language module under vim.languages.csharp

Sanity Checking

• I have updated the changelog as per my changes.
• I have tested, and self-reviewed my code.
• Style and consistency
  • I ran Alejandra to format my code (nix fmt).
  • My code conforms to the editorconfig configuration of the project.
  • My changes are consistent with the rest of the codebase.
• If new changes are particularly complex:
  • My code includes comments in particularly complex areas
  • I have added a section in the manual.
  • (For breaking changes) I have included a migration guide.
• Package(s) built:
  • .#nix (default package)
  • .#maximal
  • .#docs-html
• Tested on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin

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