Improve Zeta rating ergonomics (#21845)

This PR adds keyboard shortcuts to common interactions you might want to
do in the Zeta rating panel.

This PR also adds a way to fake inline completion requests, as well as
the test data used to create this PR, to make it easier to adjust the UI
in the future.

It also changes the status bar from the text "Zeta" to "ζ", because I
thought it looked cool.

Release Notes:

- N/A