HTML API: Allow more set_modifiable_text contents where possible.

[jonsurrell]

There are cases where ::set_modifiable_text() rejects contents that are safe.

Some cases reject based on false !== stripos( $plaintext_content, "</{$tag_name}" ) in the text to be set:

• ​https://github.com/WordPress/wordpress-develop/blob/b21fdb73bfd67730d90ec2895f9372e42e26655c/src/wp-includes/html-api/class-wp-html-tag-processor.php#L4083-L4097
• ​https://github.com/WordPress/wordpress-develop/blob/b21fdb73bfd67730d90ec2895f9372e42e26655c/src/wp-includes/html-api/class-wp-html-tag-processor.php#L4117

Those cases could all check for a case-insensitive match like "</{$tag_name}" immediately followed by one of the characters in " \t\f\r\n/>".

​See this thread for details.

WP_HTML_Tag_Processor::set_modifiable_text() rejects any content containing </ followed by the element's own tag name, even where that sequence cannot close the element. This relaxes the check to match the tokenizer.

What the problem was:

• A plain stripos() substring test rejected safe text such as </xmp-tag> in an XMP element, or </scriptx> in a SCRIPT element with an unrecognized content type.

What the fix does:

• Requires the tag name to be terminated by one of , \t, \f, \r, \n, /, or > before rejecting the update, at both cited sites.

Approach and why:

• This is the check the tokenizer itself performs in the RAWTEXT / script-data end tag name states, so accepted text is exactly the text that round-trips. \f is retained even though WP's tokenizer does not currently treat it as a terminator, because browsers do.

Trac ticket: https://core.trac.wordpress.org/ticket/65824

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[jonsurrell]

The PR at 91e9ed72f3d58de601c2c42ad670d8bb050b6ecf seems ready. I'll refrain from committing until the current 7.1 release is settled.

[jonsurrell]

In 63513:

HTML API: Allow raw text which cannot close its own element.

Developed in: ​https://github.com/WordPress/wordpress-develop/pull/12914

Props khokansardar, jonsurrell, shailu25.
Fixes #65824.