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/src/jj.rs

@@ -0,0 +1,5 @@
+mod jj_repository;
+mod jj_store;
+
+pub use jj_repository::*;
+pub use jj_store::*;