Adds PropertySetter::supportsChildEditing() and ResourceDefinitionValidator,
so a relationship declared writeable() where linkable() was meant is reported
at build time instead of failing a write with an unrelated-sounding
"Invalid property" error. ChildNotEditableException replaces that generic
error on the runtime path.

Minor rather than major: PropertySetter gains an interface method, but every
shipped implementation extends CatLab\Charon\Resolvers\PropertySetter, which
provides it.