Add a picker for jj bookmark list (#30883)

This PR adds a new picker for viewing a list of jj bookmarks, like you
would with `jj bookmark list`.

This is an exploration around what it would look like to begin adding
some dedicated jj features to Zed.

This is behind the `jj-ui` feature flag.

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -565,6 +565,7 @@ fn main() {
         notifications::init(app_state.client.clone(), app_state.user_store.clone(), cx);
         collab_ui::init(&app_state, cx);
         git_ui::init(cx);
+        jj_ui::init(cx);
         feedback::init(cx);
         markdown_preview::init(cx);
         welcome::init(cx);