Follow-up to the discussion in #51670.

@ConradIrwin, thanks for the review!

You're right about the other editors. VS Code and JetBrains use
dedicated shortcuts for bottom panel tabs, which exactly mirrors how
`ctrl-`` already behaves in Zed, so we don't need a new keybinding.

The reason I originally proposed `cmd-shift-j` was just to match the UI
pattern of the Debugger, which clearly shows `cmd-shift-d` on hover.
Because of a minor UI discoverability bug, the Terminal button's tooltip
was completely blank, which made it look like it just lacked a shortcut
entirely.

It turns out the Terminal button's internal `toggle_action()` method was
just telling the UI to look up `ToggleFocus` instead of
`terminal_panel::Toggle` (which is what `ctrl-`` is actually bound to).

I've updated this branch with a simple one-word fix that just points the
`toggle_action` to the correct keymap. The existing `ctrl-`` shortcut
now shows up perfectly in the hover tooltip. Good catch on the existing
shortcuts, keeps this much cleaner.

Co-authored-by: Michael Smolkin <msmolkin@users.noreply.github.com>