Correction of ExcelModuleInterface

[Haselnussbomber]

The internal field ExcelModule in UIModule was incorrect. ExcelModule inherits from Common::Component::Excel::ExcelModuleInterface, but the field is storing a pointer to Component::Excel::ExcelModuleInterface (without the Common:: prefix). It's a different thing.

Changes:

• UIModule vf2 was renamed from GetExcelModule to GetExcelModuleInterface, though that is still confusing since there are 2 of those.
• The other ExcelModuleInterface (with Common prefix) was added as struct and ExcelModule now inherits from it.
• GetLanguage() vfuncs were added to both interfaces with their respective enums. (I'm not sure if the ExcelLanguage enum with the None value should be living somewhere else, like in ExdModule or ExcelSheet.)

The values for the ExcelLanguage enum were taken from Lumina: https://github.com/NotAdam/Lumina/blob/master/src/Lumina/Data/Language.cs#L5