force_balance_tags breaks new gallery block

[glendaviesnz]

The refactored gallery block uses a nested <figure><figure></figure></figure> structure, as recommended by w3c for image collections.

If a site has the use_balanceTags option set to 1, then the Gallery block breaks on save as force_balance_tags restructures it from <figure><figure></figure></figure> to <figure></figure><figure></figure>.

This is because figure is not listed as a directly nestable element at https://core.trac.wordpress.org/browser/trunk/src/wp-includes/formatting.php#L2485

The refactored gallery block uses a nested <figure><figure></figure></figure> structure, as recommended by w3c for image collections.

If a site has the use_balanceTags option set to 1, then the Gallery block breaks on save as force_balance_tags restructures it from <figure><figure></figure></figure> to <figure></figure><figure></figure>.

This PR adds figure as as a directly nestable element, which it is under the current html specs.

Trac ticket: https://core.trac.wordpress.org/ticket/54130

@glendaviesnz There are some unit tests for balanceTags. It looks like all that needs to be done is update this array for this change to be covered:
​https://github.com/WordPress/wordpress-develop/blob/de822e8fb02e65bea74e9fa75b889461ef234bb3/tests/phpunit/tests/formatting/balanceTags.php#L8-L16

@glendaviesnz There are some unit tests for balanceTags. It looks like all that needs to be done is update this array for this change to be covered:
​https://github.com/WordPress/wordpress-develop/blob/de822e8fb02e65bea74e9fa75b889461ef234bb3/tests/phpunit/tests/formatting/balanceTags.php#L8-L16

There are some unit tests for balanceTags. It looks like all that needs to be done is update this array for this change to be covered:

Thanks, have update the test.

[hellofromTonya]

​PR 1678 is ready for commit.

[hellofromTonya]

Assigning to me for commit review and prep.

[hellofromTonya]

There's possibility the new v2 gallery block may be reverted during Beta (before RC) due to mobile instability with migrating from v1 to v2 gallery blocks. Noting here in the event it is reverted. However in discussions with @glendaviesnz, moving v2 gallery block to 6.0 should not result in structural changes. Thus, this patch can be committed as it will be needed regardless if v2 does move to 6.0.

Committed via changeset https://core.trac.wordpress.org/changeset/52188.

[hellofromTonya]

Incorrectly marked the ticket number in the commit. It belongs to this ticket.

In 52188:

Formatting: Add additional support for single and nestable tags in force_balance_tags().

Adds track and wbr support for single tags.

Adds article, aside, details, figure, and section for nestable tags.

Updates tests.

Follow-up to [5805], [21828], [45929].

Props glendaviesnz, costdev, talldanwp, ramonopoly, sergeybiryukov.
Fixes #50225.