This PR updates the `zed_extension_api` to v0.1.0 for the extensions
that live in this repo.
The changes in that version of additive, so none of the extensions need
to change their usage in order to upgrade.
Release Notes:
- N/A
This PR extracts C# support into an extension and removes the built-in
C# support from Zed.
Tested using a Nix shell:
```
nix-shell -p dotnet-sdk omnisharp-roslyn
```
Release Notes:
- Removed built-in support for C#, in favor of making it available as an
extension. The C# extension will be suggested for download when you open
a `.cs` file.
