[ticket/12841] Allow extensions to position new config vars
* VSEphpbb/ticket/12841:
[ticket/12841] Make config position default to after
[ticket/12841] Update comments in tests making after default
[ticket/12841] prefix function name with phpbb_
[ticket/12841] Add a tests for the new function
[ticket/12841] Fix white space issues reported by sniffer
[ticket/12841] Allow extensions to position new config vars