Gallery shortcode fallback to respect post_gallery filter

[antpb]

Description

This PR is an first pass attempt at providing a fallback to the gallery shortcode that will allow support for the post_gallery filter. More information can be found here: #1451

This PR conditionally registers the callback function based on has_filter( 'post_gallery')
I'm opening this PR to have discussion on this approach.

[antpb]

Currently having issues getting the render callback function to register in register_block_type. I've commented out the has_filter check as I figure out why the function is not registering

[gziolo]

What was needed in this issue was a way to parse the ids from the Gutenberg gallery. This has been achieved in #11540 so I'll be closing this.

@antpb I noticed your comment that the parent issue was resolved. I'm closing this PR. Feel free to reopen if you think that it still needs to be implemented.

[tyrann0us]

What was needed in this issue was a way to parse the ids from the Gutenberg gallery. This has been achieved in #11540 so I'll be closing this.
— @antpb in #1451 (comment)

@gziolo, if I understand it correctly, the ability to parse IDs from the gallery still doesn't allow plugin/theme authors to change the markup of a gallery created with the "Gallery" block using the post_gallery filter or any other way. In my opinion, neither #1451 nor this PR (#10392) should have been closed.

[gziolo]

It's perfectly fine to use the following logic in the plugin and completely override the output which JS code generated with save function:

register_block_type(
    'core/gallery',
    array(
        'render_callback' => 'my_plugin_render_block_gallery',
    )
);

Do you see any issue with such approach? By the way, the long term plan is to have all blocks registered on the server as proposed in #13693. When that happens it should be possible to be able to filter all settings that get registered using register_block_type (unless it is possible as of today - I don't know to be honest - it's part of WordPress core codebase).