lsp: Serialize LSP notifications on background threads (#39403)

This should reduce hiccups when opening large files.

Release Notes:

- N/A

crates/editor/src/test/editor_lsp_test_context.rs

@@ -440,7 +440,7 @@ impl EditorLspTestContext {
     }
 
     pub fn notify<T: notification::Notification>(&self, params: T::Params) {
-        self.lsp.notify::<T>(&params);
+        self.lsp.notify::<T>(params);
     }
 
     #[cfg(target_os = "windows")]