HTML API: Join text nodes on invalid-tag-name boundaries.

A fix was introduced to the Tag Processor to ensure that contiguous text
in an HTML document emerges as a single text node spanning the full
sequence. Unfortunately, that patch was marginally over-zealous in
checking if a "<" started a syntax token or not. It used the following:

```
if ( 'A' <= $c && 'z' >= $c ) { ... }
```

This was based on the assumption that the A-Z and a-z letters are
contiguous in the ASCII range; they aren't, and there's a gap of
several characters in between. The result of this is that in some
cases the parser created a text boundary when it didn't need to.
Text boundaries can be surprising and can be created when reaching
invalid syntax, HTML comments, and more hidden elements, so
semantically this wasn't a major bug, but it was an aesthetic
challenge.

In this patch the check is properly compared for both upper- and
lower-case variants that could potentially form tag names.

```
if ( ( 'A' <= $c && 'Z' >= $c ) || ( 'a' <= $c && 'z' >= $c ) ) { ... }
```

This solves the problem and ensures that contiguous text appears
as a single text node when scanning tokens.

Follow-up to [57489]
Props dmsnell, jonsurrell
See Core-60385

src/wp-includes/html-api/class-wp-html-tag-processor.php

@@ -1528,20 +1528,26 @@ private function parse_next_tag() {
 
 			if ( $at > $was_at ) {
 				/*
-				 * A "<" has been found in the document. That may be the start of another node, or
-				 * it may be an "ivalid-first-character-of-tag-name" error. If this is not the start
-				 * of another node the "<" should be included in this text node and another
-				 * termination point should be found for the text node.
+				 * A "<" normally starts a new HTML tag or syntax token, but in cases where the
+				 * following character can't produce a valid token, the "<" is instead treated
+				 * as plaintext and the parser should skip over it. This avoids a problem when
+				 * following earlier practices of typing emoji with text, e.g. "<3". This
+				 * should be a heart, not a tag. It's supposed to be rendered, not hidden.
+				 *
+				 * At this point the parser checks if this is one of those cases and if it is
+				 * will continue searching for the next "<" in search of a token boundary.
 				 *
 				 * @see https://html.spec.whatwg.org/#tag-open-state
 				 */
 				if ( strlen( $html ) > $at + 1 ) {
 					$next_character  = $html[ $at + 1 ];
-					$at_another_node =
+					$at_another_node = (
 						'!' === $next_character ||
 						'/' === $next_character ||
 						'?' === $next_character ||
-						( 'A' <= $next_character && $next_character <= 'z' );
+						( 'A' <= $next_character && $next_character <= 'Z' ) ||
+						( 'a' <= $next_character && $next_character <= 'z' )
+					);
 					if ( ! $at_another_node ) {
 						++$at;
 						continue;