Blocks: Introduce helper function to retrieve hooked blocks

[gziolo]

Trac ticket: https://core.trac.wordpress.org/ticket/59383

In order to implement Block Hooks (see #59313), we added block_hooks field to the WP_Block_Type class, as well as to block registration related functions. In this follow-up, new helper function gets introduced that is going to compute the list of hooked blocks by other registered blocks for a given block type.

Extracted from #5158 and covered with unit tests.

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[ockham]

Looking good overall, just one request regarding tests.

For the other PRs we've filed for block hooks, I've actually filed separate tickets; it'd be great if we could do the same here 😊 (I can do that if you're busy with other things.)

Finally, just noting that I agree that we should eventually cache get_hooked_blocks as we discussed; but let's keep that for a follow-up.

[gziolo]

For the other PRs we've filed for block hooks, I've actually filed separate tickets; it'd be great if we could do the same here 😊 (I can do that if you're busy with other things.)

Feel free to open another ticket. Otherwise, I'm fine referencing the existing ticket that covers the block hooks feature.

[gziolo]

I addressed the feedback, and I will open a ticket for this functionality now that I have some additional time.

[gziolo]

The ticket is https://core.trac.wordpress.org/ticket/59383.

[ockham]

Thank you very much! Let's update the two @ticket references in the tests to use the new number, and it's ready to go 🎉

[gziolo]

Committed with https://core.trac.wordpress.org/changeset/56610.