language: Spawn language servers on background threads (#44631)

Closes https://github.com/zed-industries/zed/issues/39056

Leverages a new `await_on_background` API that spawns the future on the
background but blocks the current task, allowing to borrow from the
surrounding scope.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/copilot/src/copilot.rs

@@ -516,7 +516,8 @@ impl Copilot {
                 None,
                 Default::default(),
                 cx,
-            )?;
+            )
+            .await?;
 
             server
                 .on_notification::<StatusNotification, _>(|_, _| { /* Silence the notification */ })