Move the facets part to an optional submodule

Problem/Motivation

This module depends on https://www.drupal.org/project/facets → which is rather heavy.

For sites that don't use facets (for example to the select-a11y on forms), this is dependency is unnecessary.

Proposed resolution

Move the `facets` part of this module to a submodule.

Remaining tasks

Create MR to move the code to submodule.

User interface changes

Extra module to enable so this is not backward compatible. Sites depending on the facets part of this module would need to be updated to enable the submodule.