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/languages/src/tailwind.rs

@@ -2,7 +2,7 @@ use anyhow::{anyhow, Result};
 use async_trait::async_trait;
 use collections::HashMap;
 use futures::StreamExt;
-use gpui::AppContext;
+use gpui::AsyncAppContext;
 use language::{LanguageServerName, LspAdapter, LspAdapterDelegate};
 use lsp::LanguageServerBinary;
 use node_runtime::NodeRuntime;
@@ -107,12 +107,16 @@ impl LspAdapter for TailwindLspAdapter {
         })))
     }
 
-    fn workspace_configuration(&self, _workspace_root: &Path, _: &mut AppContext) -> Value {
-        json!({
+    async fn workspace_configuration(
+        self: Arc<Self>,
+        _: &Arc<dyn LspAdapterDelegate>,
+        _cx: &mut AsyncAppContext,
+    ) -> Result<Value> {
+        Ok(json!({
             "tailwindCSS": {
                 "emmetCompletions": true,
             }
-        })
+        }))
     }
 
     fn language_ids(&self) -> HashMap<String, String> {