Add-ons Standard Library 1.21.13

Required for all XF2.1+ compatible add-ons.

A number of helper utilities designed to ease add-on development, does not have any direct user-facing changes. See README.md for additional details.

Uploaded to aid dependency tracking and allow updating without updating every individual add-on.

Warning: this add-on is incompatible which cause site-disabling errors with older versions of the following add-ons;