Convert custom command Key fields to Keybinding

CustomCommand.Key and CustomCommandMenuOption.Key are user-configured
keybindings just like the built-in ones. Converting them to the Keybinding type
lets a user assign multiple keys to the same custom command, e.g. `key: [a, b]`,
the same way they would for any other keybinding.

The validator iterates over the elements rather than checking a single string,
the binding registration goes through GetValidatedKeyBindingKeys to register
every alternate, and the existing error messages use .String() so a multi-key
binding renders sensibly.

CustomCommandPrompt.Key (a form field name, not a keybinding) stays a plain
string.