overlay method
Returns a copy where this set takes precedence over base: at each level,
this map's entries win per key, and base's remaining entries are kept.
Implementation
SchemaTypeOverrides overlay(SchemaTypeOverrides base) => SchemaTypeOverrides(
byColumn: {...base.byColumn, ...byColumn},
byNative: {...base.byNative, ...byNative},
byCanonical: {...base.byCanonical, ...byCanonical},
);