Change Module Package Category to `User Interface` to be More Specific than `Custom`

Currently the module is categorized in the "Custom" category on the Extend directory (modules page). This is left over from initial development.

Other Gin ecosystem modules are most often either uncategorized (represented by the "Other" category) or, in the case of Gin Layout Builder, categorized as "User Interface".

"User Interface" makes the most sense to me.