Reduce specificity of global styles and layout selectors

[isabel_brison]

In order to enable the extension of block style variations to allow for section styling as described here: ​https://github.com/WordPress/gutenberg/issues/57537, it is necessary to reduce the specificity of global styles CSS output.

The following PRs need to be synced to core:

​https://github.com/WordPress/gutenberg/pull/60106
​https://github.com/WordPress/gutenberg/pull/60228
​https://github.com/WordPress/gutenberg/pull/61340
​https://github.com/WordPress/gutenberg/pull/61638

Trac ticket: https://core.trac.wordpress.org/ticket/61165

Syncs changes from ​https://github.com/WordPress/gutenberg/pull/60106, ​https://github.com/WordPress/gutenberg/pull/60228 and ​https://github.com/WordPress/gutenberg/pull/61340.

Todo: update Tests_Theme_wpThemeJson test strings.

Just realised that changes from #6489 are likely to conflict with these, especially the test string updates, so I might hold off on updating this one until #6489 is committed.

PHP tests are updated, so marking this ready for review!

There are some proposed changes to specificity in ​https://github.com/WordPress/gutenberg/pull/61638 and a ​general consensus to proceed with those changes for WP6.6.

Given the tight timeline until the 6.6 beta I've put together a ​separate draft PR bringing together all the specificity changes from the following PRs:

• ​https://github.com/WordPress/gutenberg/pull/60106
• ​https://github.com/WordPress/gutenberg/pull/60228
• ​https://github.com/WordPress/gutenberg/pull/61340
• ​https://github.com/WordPress/gutenberg/pull/61638

Once they settle that could be rolled into this PR or the core ticket updated to point to the alternate backport.

Closing in favour of #6633.

To make overriding core styles easier and enable the extension of block style variations as a means for section styling, it is necessary to limit the specificity of global styles CSS output.

Further context can be found in: ​https://github.com/WordPress/gutenberg/issues/57537

Syncs changes from:

• ​https://github.com/WordPress/gutenberg/pull/60106
• ​https://github.com/WordPress/gutenberg/pull/60228
• ​https://github.com/WordPress/gutenberg/pull/61340
• ​https://github.com/WordPress/gutenberg/pull/61638

Trac ticket: https://core.trac.wordpress.org/ticket/61165

@tellthemachines and @andrewserong, if you get the chance, it would be great if you could confirm this backport covers all it should from previous zero-specificity work and the closed PR: ​https://github.com/WordPress/wordpress-develop/pull/6522

Thanks for the reviews and nudge to get the since/ticket comments in place. That's been done now.

I've also given this a rebase and updated the new background image test for theme.json.

[isabel_brison]

In 58241:

Editor: level global styles specificity at 0-1-0.

Adjusts the block level global styles selectors so they have a consistent specificity of 0-1-0, and adjusts base and layout global style selectors to their minimum required specificity.

Props aaronrobertshaw, isabel_brison, andrewserong, mukesh27.
Fixes #61165.

Committed in r58241.

Backports the default block style variation styles to the core theme.json file after the general specificity reduction of global styles in ​https://github.com/WordPress/gutenberg/pull/61638.

To test:

1. Prior to checking out this branch, visit the site editor and edit the home page with TT4 active
2. Select the button and switch its block style to Outline. Note the appearance doesn't change.
3. Checkout this branch and repeat the steps. The Outline style should take effect.

Trac ticket: https://core.trac.wordpress.org/ticket/61165

[isabel_brison]

In 58393:

Editor: Fix specificity of core block style variations.

Adds styles for default block style variations to core theme.json so they can override core element styles where needed.

Props aaronrobertshaw, isabel_brison.
Follows r58241.
See #61165.

Committed in r58393.