Convert Version to a sealed class

[manueldidonna]

Version is a class that should used only to retrieve the resources of specific games (pokemon species, stats, abilities, forms) and not to rule the editing logic. Library users won't ever need to implement custom version classes

sealed class Version {
    val index: Int
    data class FirstGeneration(val isYellow: Boolean) : Version() {
        override val index: Int = 1
    }
}

[manueldidonna]

Closed with bcf19b2d473553b4900f2d4280eb2eccfb88f507