Add support to Union Type

[miku1958]

Add support to union type like string | boolean
Swift uses enum,

public enum UnionType: Codable {
  case string(_ value: String)
  case double(_ value: Double)
  case bool(_ value: Bool)
  case numEnum(_ value: NumEnum)
  case defaultEnum(_ value: DefaultEnum)
}

Kotlin uses sealed class.

sealed class UnionType(val value: Any) {
    data class StringValue(val value: String) : UnionType()
    data class FloatValue(val value: Float) : UnionType()
    data class BooleanValue(val value: Boolean) : UnionType()
    data class NumEnumValue(val value: NumEnum) : UnionType()
    data class DefaultEnumValue(val value: DefaultEnum) : UnionType()
}

Old UnionType is renamed to LiteralType

[Foolment]

I feel ok to merge

[Foolment]

There should be more types, but I think it's ok right now, we can add more when we need. Thanks.