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/jj_ui/src/jj_ui.rs

@@ -0,0 +1,39 @@
+mod bookmark_picker;
+
+use command_palette_hooks::CommandPaletteFilter;
+use feature_flags::FeatureFlagAppExt as _;
+use gpui::App;
+use jj::JujutsuStore;
+use workspace::Workspace;
+
+pub fn init(cx: &mut App) {
+    JujutsuStore::init_global(cx);
+
+    cx.observe_new(|workspace: &mut Workspace, _window, _cx| {
+        bookmark_picker::register(workspace);
+    })
+    .detach();
+
+    feature_gate_jj_ui_actions(cx);
+}
+
+fn feature_gate_jj_ui_actions(cx: &mut App) {
+    const JJ_ACTION_NAMESPACE: &str = "jj";
+
+    CommandPaletteFilter::update_global(cx, |filter, _cx| {
+        filter.hide_namespace(JJ_ACTION_NAMESPACE);
+    });
+
+    cx.observe_flag::<feature_flags::JjUiFeatureFlag, _>({
+        move |is_enabled, cx| {
+            CommandPaletteFilter::update_global(cx, |filter, _cx| {
+                if is_enabled {
+                    filter.show_namespace(JJ_ACTION_NAMESPACE);
+                } else {
+                    filter.hide_namespace(JJ_ACTION_NAMESPACE);
+                }
+            });
+        }
+    })
+    .detach();
+}