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/elixir.rs

@@ -1,7 +1,7 @@
 use anyhow::{anyhow, bail, Context, Result};
 use async_trait::async_trait;
 use futures::StreamExt;
-use gpui::{AppContext, AsyncAppContext, Task};
+use gpui::{AsyncAppContext, Task};
 pub use language::*;
 use lsp::{CompletionItemKind, LanguageServerBinary, SymbolKind};
 use project::project_settings::ProjectSettings;
@@ -14,7 +14,7 @@ use std::{
     any::Any,
     env::consts,
     ops::Deref,
-    path::{Path, PathBuf},
+    path::PathBuf,
     sync::{
         atomic::{AtomicBool, Ordering::SeqCst},
         Arc,
@@ -278,16 +278,22 @@ impl LspAdapter for ElixirLspAdapter {
         })
     }
 
-    fn workspace_configuration(&self, _workspace_root: &Path, cx: &mut AppContext) -> Value {
-        let settings = ProjectSettings::get_global(cx)
-            .lsp
-            .get("elixir-ls")
-            .and_then(|s| s.settings.clone())
-            .unwrap_or_default();
+    async fn workspace_configuration(
+        self: Arc<Self>,
+        _: &Arc<dyn LspAdapterDelegate>,
+        cx: &mut AsyncAppContext,
+    ) -> Result<Value> {
+        let settings = cx.update(|cx| {
+            ProjectSettings::get_global(cx)
+                .lsp
+                .get("elixir-ls")
+                .and_then(|s| s.settings.clone())
+                .unwrap_or_default()
+        })?;
 
-        serde_json::json!({
+        Ok(serde_json::json!({
             "elixirLS": settings
-        })
+        }))
     }
 }