Replace app/app_mut on contexts with AsRef/AsMut impls

Co-Authored-By: Brooks Swinnerton <934497+bswinnerton@users.noreply.github.com>

zed/src/worktree/worktree.rs

@@ -710,7 +710,7 @@ mod test {
             let file_id = entry.entry_id;
 
             tree.update(&mut app, |tree, ctx| {
-                smol::block_on(tree.save(file_id, buffer.snapshot(), ctx.app())).unwrap()
+                smol::block_on(tree.save(file_id, buffer.snapshot(), ctx.as_ref())).unwrap()
             });
 
             let history = app