Code Modernization: Fix trigger_error() with E_USER_ERROR deprecation in Text_Diff_Op::reverse().

PHP 8.4 deprecates the use of `trigger_errror()` with `E_USER_ERROR` as the error level, as there are a number of gotchas to this way of creating a `Fatal Error` (`finally` blocks not executing, destructors not executing).
The recommended replacements are either to use exceptions or to do a hard `exit`.

This is an unmaintained external dependency; thus, the fix is made in the WP specific copy of the dependency.

As `trigger_error()` call looks to be a remnant of the PHP 4 era before a class could be declared as `abstract`, fixed by making both the method as well as the class `abstract` and removing the call to `trigger_error()`.

Ref:
* https://wiki.php.net/rfc/deprecations_php_8_4#deprecate_passing_e_user_error_to_trigger_error
* https://www.php.net/manual/en/migration80.incompatible.php

Follow-up to [7747].

Props jrf.
See #62061.

git-svn-id: https://develop.svn.wordpress.org/trunk@59106 602fd350-edb4-49c9-b593-d223f7449a82

Author: hellofromtonya

src/wp-includes/Text/Diff.php

@@ -350,15 +350,12 @@ public function Text_MappedDiff( $from_lines, $to_lines,
  *
  * @access private
  */
-class Text_Diff_Op {
+abstract class Text_Diff_Op {
 
     var $orig;
     var $final;
 
-    function &reverse()
-    {
-        trigger_error('Abstract method', E_USER_ERROR);
-    }
+    abstract function &reverse();
 
     function norig()
     {