Fix enqueueing additional styles for blocks only when rendered

[aristath]

Description

Issue reported in https://make.wordpress.org/core/2021/12/15/using-multiple-stylesheets-per-block/#comment-42320

In a block theme, additional block styles registered using the wp_enqueue_block_style function should only get printed when the block exists on a page. However, they currently always get rendered.

This PR fixes the issue so that these styles will only get printed when a block gets rendered.

How has this been tested?

Using 5.0-RC, with the twentytwentytwo theme:
Added this in the theme's functions.php file:

add_action( 'after_setup_theme', function() {
	wp_enqueue_block_style( 'core/gallery', array(
		'handle' => 'my-theme-gallery',
		'src'    => get_theme_file_uri( 'assets/blocks/gallery.css' ),
		'path'   => get_theme_file_path( 'assets/blocks/gallery.css' ),
	) );
} );

Then, I added a new assets/blocks/gallery.css file in the theme with these contents:

img {
	padding: 10px;
	background: #000;
	color: #fff;
}

Tested and confirmed that the styles only get added when the block exists.
Then switched to the twentytwentyone theme (or any other non-block theme), and confirmed that the styles get added regardless of whether the block exists on a page or not (conditional stylesheets loading should only work for block themes)

This should be backported to 5.9 👍

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[Mamaduka]

@aristath, is there a track ticket for this issue? We have to manually backport these changes since they're not part of the packages.

[aristath]

There's no trac ticket for that, it was simply reported on https://make.wordpress.org/core/2021/12/15/using-multiple-stylesheets-per-block/#comment-42320

[aristath]

@Mamaduka do you want me to create a ticket on track for this issue? Or have you already done that?

[Mamaduka]

@aristath, core ticket, and the patch would be great to ship fix with RC2.

[aristath]

Trac ticket & patch: https://core.trac.wordpress.org/ticket/54790

[Mamaduka]

The fix works as expected. Thank you, Ari.

P.S. I had to test using WP 5.8.x since plugin functions aren't loading in 5.9 RC, and used img { border: 5px solid red; } to make file existance immediately visible.

[noisysocks]

I see that these changes were already made to Core (https://core.trac.wordpress.org/changeset/52559) so just noting here to my future self that I only need to merge this into wp/5.9 for posterity and not do any actual backporting 👍