Add dotnet extension

[Gambl3r08]

Adds the .NET extension: https://github.com/Gambl3r08/zed-dotnet

What it provides

• Debugging: netcoredbg debug adapter + a dotnet locator that turns
  dotnet run tasks into build-then-debug (assembly resolved via
  dotnet msbuild -getProperty:TargetPath), plus a debugger.scm for inline
  values.
• C#: Roslyn language server (auto-downloaded from nuget.org, started with
  --autoLoadProjects), 47 semantic token rules (XML doc comments, embedded
  regex/JSON), runnables for tests/Main, tasks, snippets.
• MSBuild: one language for .csproj/.fsproj/.vbproj/.props/
  .targets/.slnx/nuget.config, with outline and package-management tasks.
• Razor: syntax, outline and indents for .razor/.cshtml (grammar embeds
  full C#); routing to Roslyn co-hosting is wired for users who supply a
  matched server pair.

Overlap with the existing csharp extension

Per the guidelines I first contributed the debugging support upstream:
zed-extensions/csharp#95 (complete, CI-green, unreviewed so far; related to
the stalled zed-extensions/csharp#43 and issue zed-extensions/csharp#12).

This extension's remaining scope — Razor, unified MSBuild, debugging integrated
with runnables and tasks — does not fit as incremental PRs to a C#-only
extension. Both can coexist; if #95 lands upstream, this extension still
provides the rest.

Tree-sitter queries are validated by cargo test against the pinned grammar
revisions; the Roslyn path and a full netcoredbg debug session have been
exercised against real projects.

• I've read CONTRIBUTING.md and followed the relevant guidance for adding or updating my extension.