Sniff: add new get_array_access_keys() utility method
Sniff: add new get_array_access_keys() utility method

... to retrieve _all_ array keys for a multi-level array access.

The original `Sniff::get_array_access_key()` method defers to the new method under the hood, however, the behaviour of that method has not changed.

paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards