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.
Now, there were basically three options:
* Silence the deprecation until PHP 9.0 and delay properly solving this until then.
This would lead to an awkward solution, as prior to PHP 8.0, error silencing would apply to all errors, while, as of PHP 8.0, it will no longer apply to fatal errors.
It also would only buy us some time and wouldn't actually solve anything.
* Use `exit($status)`.
This would make the code untestable and would disable handling of these errors via custom error handlers, which makes this an undesirable solution.
* Throw an exception.
This makes for the most elegant solution with the least BC-breaking impact.
The third option is implemented which:
* Introduces a new `Text_Exception` class.
* Starts using that in the `Text_Diff::_check()` method in all applicable places.
* Adds tests for the first two error conditions.
References:
* 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 [59070], [52978], [7747].
Props jrf.
See #62061.
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