elm: Extract to zed-extensions/elm repository (#22637)

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

Release Notes:

- N/A

docs/src/languages/elm.md

@@ -1,6 +1,6 @@
 # Elm
 
-Elm support is available through the [Elm extension](https://github.com/zed-industries/zed/tree/main/extensions/elm).
+Elm support is available through the [Elm extension](https://github.com/zed-extensions/elm).
 
 - Tree Sitter: [elm-tooling/tree-sitter-elm](https://github.com/elm-tooling/tree-sitter-elm)
 - Language Server: [elm-tooling/elm-language-server](https://github.com/elm-tooling/elm-language-server)