fix: bundle @redocly/openapi-core in action dist

The OpenAPI differ now uses @redocly/openapi-core for spec resolution
(supporting both 3.0 and 3.1). Since the action runs from its own
directory, the dynamic import fails unless redocly is bundled.

Add @redocly/openapi-core as a regular dependency so tsup bundles it
into dist/index.js. Users no longer need to install it separately.

Author: omermorad