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/feature_flags/src/feature_flags.rs

@@ -91,6 +91,12 @@ impl FeatureFlag for ThreadAutoCaptureFeatureFlag {
     }
 }
 
+pub struct JjUiFeatureFlag {}
+
+impl FeatureFlag for JjUiFeatureFlag {
+    const NAME: &'static str = "jj-ui";
+}
+
 pub trait FeatureFlagViewExt<V: 'static> {
     fn observe_flag<T: FeatureFlag, F>(&mut self, window: &Window, callback: F) -> Subscription
     where