`binding-keys` handling of prompt buffer commands

[aadcg]

Currently, bindings-keys doesn't report bound prompt buffer commands. This impacts our docs, since :nxref won't be able to show keybindings associated with prompt buffer command.

CL-USER> (nyxt:start :failsafe t)
NYXT> (binding-keys 'nyxt/mode/prompt-buffer:quit-prompt-buffer)
"UNBOUND"

When opening the prompt buffer and adding

NYXT> (binding-keys 'nyxt/mode/prompt-buffer:quit-prompt-buffer :modes (modes (current-prompt-buffer)))
"escape"

I suggest adding prompt-buffer-mode to the list of modes in binding-keys.

Additionally, when a command isn't bound to a key, :nxref should just print the command's name. I.e, "command-name" instead of "command-name ( UNBOUND )".