[#16] Use assoc array when defining scripts in PHP (#17)
If a sequential array is used for declaring the scripts to be deferred
cannot be properly extended without possibly duplicating and / or
removing existing deferred scripts, in the case of an AJAX request that
extends the Drupal settings.
In this commit, we declare the associative array with equivalent keys
and values, which will ensure that all scripts are included, and that no
duplicates are possible. We then use `Object.keys()` to convert that