[ticket/12841] Allow extensions to position new config vars

PHPBB3-12841
[ticket/12841] Fix white space issues reported by sniffer

PHPBB3-12841
[ticket/12841] Add a tests for the new function

PHPBB3-12841
[ticket/12841] prefix function name with phpbb_

PHPBB3-12841
[ticket/12841] Update comments in tests making after default

PHPBB3-12841
[ticket/12841] Make config position default to after

PHPBB3-12841
Merge pull request #2724 from VSEphpbb/ticket/12841

[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