registration: Overhaul merging of globals

registration: Overhaul merging of globals

Instead of hardcoding specific global settings in ExtensionRegistry,
create specific "merge strategies" that are used to merge globals.

Merge strategies are set for core properties in the ExtensionProcessor,
and extensions can set them for their own configuration settings using
the magic "_merge_strategy" key.

The following merge strategies are included:
* array_merge_recursive - call `array_merge_recursive` on the two arrays
* array_plus - use the "+" operator to combine arrays, preserving
               integer keys
* array_plus_2d - A version of array_plus that works on 2d arrays, used
                  for merging arrays like $wgGroupPermissions
* array_merge - call `array_merge` (default)

This changes the merging of various namespaces related settings to use
array_plus so they actually work.

Bug: T107646
Change-Id: I64cb0553864e3b78b0f203333f58bb73b86a6434