Add language_server_workspace_configuration to extension API (#10212)

This PR adds the ability for extensions to implement
`language_server_workspace_configuration` to provide workspace
configuration to the language server.

We've used the Dart extension as a motivating example for this, pulling
it out into an extension in the process.

Release Notes:

- Removed built-in support for Dart, in favor of making it available as
an extension. The Dart extension will be suggested for download when you
open a `.dart` file.

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/extensions_ui/src/extension_suggest.rs

@@ -22,6 +22,7 @@ fn suggested_extensions() -> &'static HashMap<&'static str, Arc<str>> {
             ("clojure", "cljs"),
             ("clojure", "edn"),
             ("csharp", "cs"),
+            ("dart", "dart"),
             ("dockerfile", "Dockerfile"),
             ("elisp", "el"),
             ("erlang", "erl"),