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/language/src/language.rs

@@ -197,10 +197,6 @@ impl CachedLspAdapter {
         self.adapter.code_action_kinds()
     }
 
-    pub fn workspace_configuration(&self, workspace_root: &Path, cx: &mut AppContext) -> Value {
-        self.adapter.workspace_configuration(workspace_root, cx)
-    }
-
     pub fn process_diagnostics(&self, params: &mut lsp::PublishDiagnosticsParams) {
         self.adapter.process_diagnostics(params)
     }
@@ -243,6 +239,8 @@ impl CachedLspAdapter {
 pub trait LspAdapterDelegate: Send + Sync {
     fn show_notification(&self, message: &str, cx: &mut AppContext);
     fn http_client(&self) -> Arc<dyn HttpClient>;
+    fn worktree_id(&self) -> u64;
+    fn worktree_root_path(&self) -> &Path;
     fn update_status(&self, language: LanguageServerName, status: LanguageServerBinaryStatus);
 
     async fn which(&self, command: &OsStr) -> Option<PathBuf>;
@@ -445,8 +443,12 @@ pub trait LspAdapter: 'static + Send + Sync {
         Ok(None)
     }
 
-    fn workspace_configuration(&self, _workspace_root: &Path, _cx: &mut AppContext) -> Value {
-        serde_json::json!({})
+    async fn workspace_configuration(
+        self: Arc<Self>,
+        _: &Arc<dyn LspAdapterDelegate>,
+        _cx: &mut AsyncAppContext,
+    ) -> Result<Value> {
+        Ok(serde_json::json!({}))
     }
 
     /// Returns a list of code actions supported by a given LspAdapter