Ensure that custom block element styles in theme.json are not ignored

[felixarntz]

• Add a new private function to centralize lookup logic based on theme.json path.
• Add test coverage, both an actual unit test for the new function and integration tests as part of wp_add_global_styles_for_blocks().
• Fix lookup logic to include custom block element styles and make it more resilient, as the block name should always be at a specific position.

Trac ticket: https://core.trac.wordpress.org/ticket/57868

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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.

[felixarntz]

@glendaviesnz @scruffian Would be great to get your reviews on this since you worked on the original code in WordPress/gutenberg#41446.

I worked on test coverage first. Seeing them fail on 308066b but pass on d1dcd9d shows that the updated logic correctly fixes the underlying bug.

[glendaviesnz]

This tested well for me. I added:

	"styles": {
		"blocks": {
			"core/quote": {
				"elements": {
					"cite": {
						"color": {
							"text": "blue",
							"background": "yellow"
						}
					}
				}
                         }
			"coblocks/accordion": {
				"elements": {
					"h2": {
						"color": {
							"text": "red",
							"background": "black"
						}
					}
				}
			}

and without this patch element styles for the core and custom block displayed in the editor, but only the core block element style showed in the frontend. With this patch applied both element styles displayed in the editor and frontend.

Thanks for looking at this.

[azaozz]

LGTM!

[costdev]

Thanks for the updates @felixarntz! LGTM 👍

[felixarntz]

Committed in https://core.trac.wordpress.org/changeset/56254