Allow using system node (#18172)

Release Notes:

- (Potentially breaking change) Zed will now use the node installed on
your $PATH (if it is more recent than v18) instead of downloading its
own. You can disable the new behavior with `{"node":
{"disable_path_lookup": true}}` in your settings. We do not yet use
system/project-local node_modules.

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/headless/src/headless.rs

@@ -25,7 +25,7 @@ pub struct DevServer {
 }
 
 pub struct AppState {
-    pub node_runtime: Arc<dyn NodeRuntime>,
+    pub node_runtime: NodeRuntime,
     pub user_store: Model<UserStore>,
     pub languages: Arc<LanguageRegistry>,
     pub fs: Arc<dyn Fs>,