Extend `post` and `page` features to all applicable post types

[moorscode]

We need to introduce a helper function which returns a list of post types that we need to consider in our features.

This is a basic list that can be sub-filtered in the specific implementation to allow for more fine-grained control.

We need to implement a couple of levels to make sure we only include post types which are 'workable' for the features we want to show on them.

This includes:

• Ignore noindex post types (WPSEO Settings)
• Determine what post type settings should be taken into consideration
  • Configured supported files in the post type
  • If the post type is shown in the REST API
  • Possibly more settings apply, please see https://generatewp.com/post-type/ and discuss if in doubt

We also need to implement checks to deal with an empty list of post types.

• [ ] Helper function to generate the list of post types that match our criteria
• [ ] Implement the helper function in Free
  • [ ] Make sure empty lists are considered
  • [ ] Add unit tests
• [ ] Implement the helper function in Premium
  • [ ] Make sure empty lists are considered
  • [ ] Add unit tests

Based upon https://github.com/Yoast/wordpress-seo-premium/pull/1380

[andizer]

WIP: I've created branch 8105-add-accessible-posts-helper. So this can picked up by somebody else if necessary.

[moorscode]

Related https://github.com/Yoast/wordpress-seo/issues/2751