Query Block: Allow for alternating and irregular layouts

[kjellr]

While thinking through block patterns for the Query block, I quickly ran into the fact that the block currently requires every single post to have the same layout. This is fine for standard post lists, but it's not clear how irregular or alternating layouts would be made possible yet. For example:

Alternating Layout	Irregular Layout

[mtias]

The tradeoffs here are between flexibility, clarity, and ease of creation. For "irregular" layouts, the easiest would probably be to use separate queries with offsets, or even individual posts (latest post with offset 1, latest post with offset 2, etc). For "alternating" layout, it seems like it should be among the options you choose from (like grid or masonry).

We could also have a template per item, or per condition (like 2n), but it can get extremely convoluted quickly. Any operation like that feels like it would need a bit more of a focused environment like the one proposed in #26190.

cc @jeffersonrabb in case you have thoughts on what has felt most intuitive from your work on post-list-blocks.

[aristath]

Since alternative/irregular layouts like these will be pretty theme-specific, this could be a block-style...
The theme could for example do something like this:

register_block_style(
	'core/group',
	[
		'name'         => 'alternating-columns',
		'label'        => __( 'Alternating', 'my-theme' ),
		'inline_style' => '
			.wp-block-group.is-style-alternating > .wp-block-columns:nth-child(odd),
			.wp-block-group.is-style-alternating > .wp-block-group__inner-container > .wp-block-columns:nth-child(odd) {
				flex-direction: row-reverse;
			}',
	]
);

[mtias]

@kjellr this seems the same as #25462. Going to close so that we can consolidate conversation there.