Goal: Implement the `StringToUnion` type. Type take `string` argument. The output should be a union of input letters. For example: Source challenge: https://github.com/type-challenges/type-challenges/tree/main/questions/00531-medium-string-to-union Reference explanation: https://github.com/ghaiklor/type-challenges-solutions/blob/main/en/medium-string-to-union.md