[WIP] Move taxonomy facets to a separate class + Meta fields

[felipeelia]

Description of the Change

This PR is a revamp of #2683, organizing things a bit more before making any change.

Related to https://github.com/10up/ElasticPress/issues/1440 (it does not close the issue because the range implementation should be done in another PR)

How to test the Change

Changelog Entry

Added: Support for meta fields in Facet widgets.

Credits

Props @felipeelia

Checklist:

• [ ] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.