Elements: Avoid specificity bump for top-level element-only selectors

[aaronrobertshaw]

This PR backports the PHP changes from WordPress/gutenberg#63403

These changes prevent issues caused by a bump in CSS specificity for top-level element-only global element styles. See the linked PR and original issue for further details.

Trac ticket: https://core.trac.wordpress.org/ticket/61630
Trac ticket: https://core.trac.wordpress.org/ticket/61660

Testing Instructions

Please note: JS package updates are required for the issue to be resolved in the editor. When testing, make the relevant changes in the editor and then confirm correct styles on the frontend

1. Edit a page via the site editor and add a test nav structure/markup using custom HTML block

<header>
  <nav>
     <ul>
       <li>
         <a>Test Link</a>
       </li>
     </ul>
  </nav>
</header>

2. Test custom global element styles including hover styles; Global Styles > Colors > Link
3. Add custom css using element only selector header nav ul li a. You'll need to toggle the custom HTML block preview for it to pick up the custom CSS.

header nav ul li a {
    color: red;
    text-decoration: none;
}

4. Test global block type element styles; Global Styles > Blocks > Group > Elements > Link
5. Test element style overrides on block instances
6. Define multiple block style variations that define both styles.elements and styles.blocks.core/group.elements.link styles. The following snippets can be used to create theme.json partial files within the theme's /styles directory which will then be registered as block style variations.

Example Block Style Variation Partials

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 3,
	"title": "Custom Links",
	"slug": "custom-links",
	"blockTypes": [ "core/group" ],
	"styles": {
		"elements": {
			"link": {
				"color": {
					"text": "crimson"
				},
				":hover": {
					"color": {
						"text": "darkmagenta"
					}
				}
			},
			"button": {
				"color": {
					"background": "purple",
					"text": "white"
				},
				":hover": {
					"color": {
						"background": "darkorchid"
					}
				}
			}
		},
		"blocks": {
			"core/group": {
				"elements": {
					"link": {
						"color": {
							"text": "dodgerblue"
						},
						":hover": {
							"color": {
								"text": "darkseagreen"
							}
						}
					},
					"heading": {
						"color": {
							"text": "teal"
						}
					}
				}
			}
		}
	}
}

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 3,
	"title": "Alt Links",
	"slug": "alt-links",
	"blockTypes": [ "core/group" ],
	"styles": {
		"elements": {
			"link": {
				"color": {
					"text": "green"
				},
				":hover": {
					"color": {
						"text": "lime"
					}
				}
			}
		},
		"blocks": {
			"core/group": {
				"elements": {
					"link": {
						"color": {
							"text": "coral"
						},
						":hover": {
							"color": {
								"text": "brown"
							}
						}
					}
				}
			}
		}
	}
}

7. Ensure the block style variation element styles are capped at 0-1-0 specificity
8. Test nesting block style variations and that the correct element styles are applied
9. Ensure block instance element style override the block style variation's element styles
10. Give other elements e.g. heading and cite a test, they should not have their selectors wrapped unless the style includes a pseudo selector
11. Give button and caption a run as well. Their selectors should be wrapped in :root :where() at all times.

Screenshots

Before	After

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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.

[github-actions]

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Props aaronrobertshaw, andrewserong, noisysocks.

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[github-actions]

Test using WordPress Playground

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For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[andrewserong]

This is testing well for me. At first I ran into a bug that turns out is unrelated to this change, and has a POC fix over in #7017. With that applied locally, the testing steps are working nicely for me on the site frontend, and the code change matches what merged in Gutenberg over in WordPress/gutenberg#63403

[noisysocks]

Applied the diff of this PR to the 6.6 branch and followed the testing instructions. Looks ok 👍

[noisysocks]

Committed to trunk in r58749 and re-opened https://core.trac.wordpress.org/ticket/61660 to request backport to 6.6 branch.