Added CodexJavaMode for great Java excitements.

First up: codex-method-override which asks for the name of a method (with savvy
completion) and inserts its signature and empty body, ready for implementation.

Right now the formatting is hard coded, but I'll think about how I want to
allow one to customize that. That rabbit hole goes deep. Maybe better to wire
into an external code formatter and then call that on the region when we're
done.