Ensure valid HTML is properly processed by refining regex handling

[co6x0]

Ensures valid HTML is worked correctly by wptexturize(), wp_html_split(), etc.
I started working on this PR when I noticed that using TailwindCSS children selector would break the layout of block theme (also reported in Trac ticket: 57381).

I have identified a problem with the regular expression defined in _get_wptexturize_split_regex() used in wptexturize().
This problem seemed to be affecting get_the_block_template_html() and causing the block theme layout collapse described above.
Changing this regex fixes the layout issue.

Also, wp_html_split() uses almost the same regex.
Other trac tickets caused by this function will also be fixed by updating to a similar regex.

According to the HTML reference at html.spec.whatwg.org, attribute values can contain a variety of characters.
With this in mind, I have modified the regex to exclude matching characters within quotation marks.
This fixes the misplacement of GREATER-THAN SIGN(>) and prevents other valid HTML structures from being mishandled.

I've included tests to cover these changes in tests/phpunit/tests/formatting/wpTexturize.php and tests/phpunit/tests/formatting/wpHtmlSplit.php. If there's anything I've missed, please let me know.

Trac ticket: https://core.trac.wordpress.org/ticket/57381
Trac ticket: https://core.trac.wordpress.org/ticket/45387
Trac ticket: https://core.trac.wordpress.org/ticket/43457

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[github-actions]

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[co6x0]

Added commit.
Removed tranformation of & to & in HTML attribute values modified by https://core.trac.wordpress.org/ticket/35008.

This ticket seems to have been created because the W3C HTML Validator found it to be invalid HTML, but as of now, the & in the URL is valid.

[co6x0]

These issues in the Gutenberg repository would also be resolved.

• Classic block missing content in Preview gutenberg#14861
• Characters for greater than and less than are not encoded as entities in attribute values gutenberg#15636

[dmsnell]

howdy! just wanted to stop by and mention that I've been exploring updating these same functions using the HTML API, which provides a full spec-compliant parse of the HTML stream.

You can find some rough notes on the broader roadmap

[github-actions]

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[co6x0]

@dmsnell
Thank you for letting me know.
Handling this with regular expressions has been challenging, so it would be wonderful if we could address it using the HTML API.

Please let me know if there's anything I can help with!
I will close this PR now.