HTML API: Ensure set_modifiable_text includes TEXTAREA leading newlines.

[sirreal]

Detect cases where ::set_modifiable_text() would omit a leading newline from its input and adjust accordingly. This is done by adding an additional leading newline (that is ignored by HTML parsers) in case a leading newline is detected in the input.

This follows the HTML parsing rules for TEXTAREA elements that ignores a single U+000A LINE FEED character immediately following the open tag. It also respects the guidelines on newline normalization, so a U+000D CARRIAGE RETURN also triggers the extra newline.

This PR only handles TEXTAREA, where the ::set_modifiable_text() behavior is simpler. See #10879 (review).

This is a partial fix for the ticket and is extracted from #10879.

The ::set_modifiable_text() method on WP_HTML_Tag_Processor and WP_HTML_Processor will fail to include a leading newline in the provided plaintext content on the first text of TEXTAREA, PRE, and LISTING elements.

This is due to special rules for these elements that cause a single leading newline to be ignored immediately following the open tag.

$html_processor = WP_HTML_Processor::create_fragment('<textarea></textarea>');
$html_processor->next_token();
$html_processor->set_modifiable_text( "\nAFTER NEWLINE" );
var_dump( $html_processor->get_modifiable_text() );
// string(13) "AFTER NEWLINE"
echo $html_processor->get_updated_html();
/* Prints:
<textarea>
AFTER NEWLINE</textarea>
*/

Note that the newline is present in the updated HTML, however the HTML parsing rules cause the leading newline to be ignored.

When rendered in the browser, this TEXTAREA element will report its .textContent as AFTER NEWLINE (with no leading newline) and the rendered element shows AFTER NEWLINE on the first line of the input box.

In order for the TEXTAREA to begin with a newline in its content, an additional newline must be included.

This is closely related to #64607.

Trac ticket: https://core.trac.wordpress.org/ticket/64609

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