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/prettier/src/prettier.rs

@@ -320,6 +320,7 @@ impl Prettier {
             Default::default(),
             &mut cx,
         )
+        .await
         .context("prettier server creation")?;
 
         let server = cx