Add external Racket extension to add LSP support for Racket

[Reilithion]

Addresses #1828. With this, it is now possible to use racket-langserver in Zed, providing the benefits of the Racket language server.

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[maxdeviant]

Thank you for the PR!

I think we'd like to see the language server support added to the existing Racket extension: https://github.com/zed-extensions/racket