Fix: Incorrect selector generated by append_to_selector method

[t-hamano]

Fixes: #48393
Fixes: #48706

Related to: #47833

What?

This PR fixes the problem that the append_to_selector() method of the WP_Theme_JSON_Gutenberg class does not generate the correct selector.

Why?

In #47833, a process has been added to check if the selector contains a comma to avoid unwanted implide/explode. However, the parameters of the str_contains function to check for it appears to be reversed.

Testing Instructions

Confirm that the correct selector is generated in the two issues that this PR fixes.

Testing for #48393 (Button Block: Style for pseudo-elements has priority)

The pseudo-element selector should be correctly generated and the background color of the button should change to the expected color.

Before

After

Testing for #48706 (Heading in selectors aren't set correctly at the block level)

• Enable EmptyTheme and overwrite theme.json with the code described in this comment.
• Insert a group block in the post.
• Confirm that the global inline style is outputting the correct selector.

Before

After

[t-hamano]

I believe this PR would change the performance measurements reported in this comment, but I wasn't sure how to test it. I would appreciate it if you could perform the performance measurement again 🙏

[github-actions]

Flaky tests detected in e08279b.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4336234328
📝 Reported issues:

• [Flaky Test] allows rotating using the crop tools #36904 in /test/e2e/specs/editor/blocks/image.spec.js
• [Flaky Test] default to the only existing classic menu if there are no block menus #48033 in /test/e2e/specs/editor/blocks/navigation.spec.js
• [Flaky Test] should navigate block hierarchy using only the keyboard #47640 in specs/editor/various/block-hierarchy-navigation.test.js
• [Flaky Test] should navigate using the list view sidebar #47001 in specs/editor/various/block-hierarchy-navigation.test.js

[oandregal]

Backporting as of WordPress/wordpress-develop#4412

[felixarntz]

@oandregal I suggest we wait for this until #50266 to backport? Or is there any rush in backporting this now?

[oandregal]

It's fine to wait. I just wanted to have this linked somewhere. As a general practice, but also, we don't forget the proper attribution to t-hamano: see WordPress/wordpress-develop#4412 (comment) :)

[spacedmonkey]

Backported https://core.trac.wordpress.org/changeset/55907