Goal: Implement a type `IsUnion`, which takes an input type `T` and returns whether `T` resolves to a union type. For example: Source challenge: https://github.com/type-challenges/type-challenges/tree/main/questions/01097-medium-isunion Reference explanation: https://github.com/ghaiklor/type-challenges-solutions/blob/main/en/medium-isunion.md