Revisions: Add a post type-specific filter to `wp_revisions_to_keep()`

[dlh]

The attached patch proposes a post type-specific filter for wp_revisions_to_keep() to complement the wp_revisions_to_keep filter.

wp_revisions_to_keep is a perfectly capable filter; the one proposed here offers just some added developer convenience for simpler cases. For example:

add_filter( 'wp_post_revisions_to_keep', fn () => 500 );
add_filter( 'wp_page_revisions_to_keep', '__return_zero' );

[adamsilverstein]

Thanks for the patch @dlh - looks good. The only think I would add is a note on the doc block indicating that the filter also overrides the existing wp_revisions_to_keep filter when used.

In addition, some basic unit tests to confirm both of these filters works as expected would be a good addition.

[dlh]

Thanks for the review, @adamsilverstein!

51550.2.diff​ updates the docs of the new filter to note its relationship to the existing filter, and it adds tests for the behavior of each filter.