[ticket/11792] Add variable 'lang_set_ext' to event core.user_setup

To allow extensions to add global language strings just like mods can, add the
'lang_set_ext' variable to the core.user_setup event. It requires an ext_name
to be specified as well as a lang_set, and loads the specified lang_set in the
context of the extension.

PHPBB3-11792
[ticket/11792] Add performance remark to core.user_setup event PHPDoc

To prevent extension authors from loading all their translations globally, a
remark on this was added to the PHPDoc documentation of the core.user_setup
event.

PHPBB3-11792
[ticket/11792] Add functional test for var lang_set_ext of core.user_setup

To ensure that the new lang_set_ext variable available with the
core.user_setup event works properly, a functional test was added. It
overwrites the value of the 'SKIP' language key, which is assumed to remain
in use for some time to come.

PHPBB3-11792
Merge remote-tracking branch 'rechosen/ticket/11792' into develop

* rechosen/ticket/11792:
  [ticket/11792] Add functional test for var lang_set_ext of core.user_setup
  [ticket/11792] Add performance remark to core.user_setup event PHPDoc
  [ticket/11792] Add variable 'lang_set_ext' to event core.user_setup