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.
Until now every keybinding config field was a plain string. That meant a user
couldn't ask for two keys to invoke a command — the config silently accepted
only one form.
Convert every string-typed field across all 13 KeybindingXxxConfig structs to
Keybinding so the union type extends to every command. Defaults wrap their
single-key value in Keybinding{...} so the generated Config.md still renders one
scalar key per binding.
The alt fields keep their separate Binding registrations for now: this commit
does not yet introduce the merge mechanism that folds them into the main field —
that comes in a follow-up. Consumers previously calling opts.GetKeys on a string
field now call opts.GetKeys on the Keybinding, or take .String() / Keys[0] where
a single value is needed.
Adds a Keybinding.String helper for rendering, schema-generator work that
inlines the Keybinding union into each consuming property, and a unit test
covering the user-facing scalar/sequence YAML forms for quit.
The folder custom_commands contained tests for both custom commands (the ones
you configure in config.yml) and shell commands (the ones you execute at the ":"
prompt). I always found this confusing, so separate these into two different
folders.