Blocks: Add isDefault to server side block styles registry

[xavivars]

What problem does this address?

As documented here, block styles can be registered both server side (using PHP method register_block_style) and client side (using wp.blocks.registerBlockStyle).

While server-side registration is a convenient method to easily register new block styles, it lacks the ability to register a style as the default one.

As an example, something that could easily be achieved like this in JS, is not doable in PHP

wp.blocks.registerBlockStyle( 'core/heading', [ 
	{
		name: 'default',
		label: 'Default',
		isDefault: true,
	},
	{
		name: 'alt',
		label: 'Alternate',
	}
]);

What is your proposed solution?

Extend register_block_style to also accept isDefault.
(it would probably require to change core code in https://github.com/WordPress/WordPress/blob/59d32bfe24cee58554ac82573d8a69dd6b6749ab/wp-includes/script-loader.php#L2311 )

[skorasaurus]

Thanks for reporting. It's worth noting that there has been discussion on reconceptualizing what 'default' means when registering a block style and removing the isDefault property although no decision has been made.

[gziolo]

Yes, it needs an upstream change in WordPress core. We should move it to WordPress trac.

[gziolo]

Ticket in WP trac filed: https://core.trac.wordpress.org/ticket/53006. I will close this one. Let's continue there.