Store an Fs on Workspace, pass it to each Worktree

Push test-only worktree behavior down into the Fs,
via a `watch` method which provides fs events.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

zed/src/lib.rs

@@ -21,6 +21,7 @@ pub struct AppState {
     pub languages: std::sync::Arc<language::LanguageRegistry>,
     pub rpc_router: std::sync::Arc<ForegroundRouter>,
     pub rpc: rpc::Client,
+    pub fs: std::sync::Arc<dyn worktree::Fs>,
 }
 
 pub fn init(cx: &mut gpui::MutableAppContext) {