WP_Theme::is_block_theme(): Add more methods of identifying a block theme.

[costdev]

Based on the paths outlined by @anton-vlasenko. Includes unit tests. Ready for review.

Trac ticket: https://core.trac.wordpress.org/ticket/54910

[costdev]

Without trim(), parse_blocks() will interpret an empty last line as a block with blockName => NULL, which could lead to a false negative.

[Mamaduka]

questions (non-blocking): Is there a reason we're parsing the blocks? The str_contains should be enough to check the existence of the blocks.

The has_blocks uses a similar check as well - https://developer.wordpress.org/reference/functions/has_blocks/.

[costdev]

Hybrid themes may use mixed HTML (block and non-block markup). The thinking here is that parsing the blocks is a way to determine whether this is a "pure" block template or not.

The str_contains() is a quick way for us to determine if there are any blocks. If there are no blocks, we can return false and save the resources required to parse the contents more thoroughly.

[anton-vlasenko]

LGTM! Nice job on the unit tests.

[costdev]

Removed the fix for get_query_template() as this turned out to be the fix for another ticket and is more appropriate to include there instead.