emmet: Extract to zed-extensions/emmet repository (#36323)

This PR extracts the Emmet extension to the
[zed-extensions/emmet](https://github.com/zed-extensions/emmet)
repository.

Release Notes:

- N/A

docs/src/languages/emmet.md

@@ -1,5 +1,7 @@
 # Emmet
 
+Emmet support is available through the [Emmet extension](https://github.com/zed-extensions/emmet).
+
 [Emmet](https://emmet.io/) is a web-developer’s toolkit that can greatly improve your HTML & CSS workflow.
 
 - Language Server: [olrtg/emmet-language-server](https://github.com/olrtg/emmet-language-server)