Restore the ability to set $wgResourceModuleSkinStyles for 'oojs-ui-core.styles'

Restore the ability to set $wgResourceModuleSkinStyles for 'oojs-ui-core.styles'

In c8ad83310f425433791102e8a958397f2d701b65, 'oojs-ui-core.styles'
was changed to use ResourceLoaderOOUIFileModule instead of plain
ResourceLoaderFileModule. This had the unintended consequence that
ResourceLoader::isFileModule() no longer returned true for it, and
this $wgResourceModuleSkinStyles no longer worked, breaking styling
in Vector.

Additionally, the new ResourceLoaderOOUIFileModule did not respect
the skinScripts/skinStyles options passed in the module definition
(therefore also those from $wgResourceModuleSkinStyles). Merging
them turns out to be a major pain, but it can be done.

Bug: T167042
Change-Id: I7547bbe996467745c1f0b168b40f27eb95c6238d