8.0.0 Calling this function from outside a class, without any arguments, will now throw an Error.
Previously, an E_WARNING was raised and the function returned false.

Applies to both get_class() as well as get_called_class().

Refs:

• https://www.php.net/manual/en/function.get-class.php#refsect1-function.get-class-changelog
• php/php-src@213b666
• https://3v4l.org/f05rn
• https://3v4l.org/a7h7r

This adds a new sniff to detect the above, which wasn't mentioned in the PHP 8.0 generic changelog nor in the migration guide, though a note about it can be found on the functions manual pages.

Includes tests and documentation.

Related to #809