API: Include setupSkinUserCss in prop=modules for useskin mode

API: Include setupSkinUserCss in prop=modules for useskin mode

Follows-up 90c95fc7f290f, which included result of Skin::getDefaultModules
in the prop=modules list. All hardcoded modules in OutputPage and Parser
were also subsequently moved into Skin::getDefaultModules.

However, a number of modules cannot be moved there because fundamentally
Skin::getDefaultModules can only load modules via OutputPage::addModules().

For style modules, addModuleStyles() must be used.

Fortunately, there is already a centralised place for that, namely
Skin::setupSkinUserCss(). Include that in the ApiParse return as well.
That should resolve the last bit of inconsistency between ApiParse
and OutputPage when it comes to the module queue.

Bug: T140664
Change-Id: I35e2e3bbdccdd1aa2a259b8e624daa80c609ba8c