Allow loading block-styles on render (opt-in) + inlining

[aristath]

This PR backports all changes related to block styles loading from Gutenberg to core.

Trac ticket: https://core.trac.wordpress.org/ticket/50328

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[aristath]

This PR does the following:

• Adds an action in Webpack to copy the separate block-styles to wp-includes/blocks/{$block_name}/*.css
• Adds a should_load_separate_block_assets function and load_separate_block_assets filter (see https://github.com/WordPress/gutenberg/blob/b786a9416b311fc550b22a50927b930a36568dca/lib/compat.php#L16-L26). The filter defaults to false and is opt-in.
• Adds a wp_maybe_inline_styles function (see https://github.com/WordPress/gutenberg/blob/4146d19b3bd276f4d6afbc7abc373b8ada18b63a/lib/blocks.php#L201-L269)
• Tweaks the register_block_style_handle function to allow loading separate files instead of the monolithic style.css file which contains styles for all blocks
• Removes the add_action( 'enqueue_block_assets', 'wp_enqueue_registered_block_scripts_and_styles' ); hook
• Adds path data to block styles to allow inlining them - see https://core.trac.wordpress.org/ticket/52620. This was done in tandem with this PR because these 2 go hand-in-hand and doing them together is simpler

[gziolo]

This adds 260 CSS files to subfolders in src/wp-includes/blocks. It won't scale well because they can change not only when the content of files updates but also when we modify webpack configuration. In this folder, we also have block.json files and PHP files copied over from WordPress packages with webpack. The only file that is custom is inside index.php. Ideally, we should remove all files inside src/wp-include/blocks from tracking in svn and leave it to the build process. I think it was proposed by @peterwilsoncc in https://core.trac.wordpress.org/ticket/49635.

[gziolo]

In my opinion, it is nearly ready to land. There are only some minor things left from my perspective. The configuration for SVN to ignore all CSS files that webpack builds being the biggest concern.

[aristath]

Added the copied CSS files to the .gitignore list.

[gziolo]

I performed a few tests with a demo post using all combinations of two filters that @aristath recommended:

add_filter( 'separate_core_block_assets', '__return_true' );
add_filter( 'styles_inline_size_limit', '__return_zero' );

I wanted to add that most of the functionality was extensively tested in Gutenberg plugin by Ari and me a few weeks/months ago. The functionality also got some testing in the plugin and was iterated upon.

So far everything works as expected in my testing. I also plan to verify with a custom block that defines CSS in block.json files that gets registered on the server.

[gziolo]

Done with https://core.trac.wordpress.org/changeset/50836 🎉

Amazing work @aristath 👏🏻 👏🏻 👏🏻 👏🏻 👏🏻