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' );