Allow default action to use menu description

[bdarcus]

which-key is getting support for simple menus, but the default command (per here) is shown in lieu of the menu description.

See @minad's suggestion here:

https://github.com/oantolin/embark/issues/207#issuecomment-864489351

Before overwriting the RET keybinding with the current command, Embark could look up the command in the keymap. If it is a menu-item, use that. If the command does not exist, just use the plain command, which of course looks more ugly. But not much can be done about that.

[minad]

The problem here is that we need a reverse keymap lookup or one needs to scan the entire keymap for the menu items. It needs a little effort, but I think it is worth it. (EDIT: this is easier than I thought given map-keymap.)

Unrelated - we may also want to update the completing-read prompter to show the menu-item descriptions additionally to the binding and command. Maybe we are already doing this, I have to check. (EDIT: we already support this for consult narrowing)