Add Keybinding type that accepts a string or a sequence of strings

Each user-configurable keybinding is currently a single string in the YAML
config. To let users assign alternate keys to a command, introduce a Keybinding
type that decodes from either a scalar (the existing single-key form, kept for
backward compatibility and for a simpler config file) or a sequence of strings.
Marshalling collapses single-element slices back to a scalar so configs and
generated docs round-trip cleanly.

JSONSchema describes the type as a oneOf union so editors validate either form;
subsequent commits will inline the union into the generated schema and start
using Keybinding as the field type.