Extracts Keys from an Object or Tuple.
Example usage is shown below.
const T = Type.Object({
x: Type.Number(),
y: Type.Number(),
z: Type.Number()
})
const S = Type.KeyOf(T) // const S: TUnion<[
// TLiteral<'x'>,
// TLiteral<'y'>,
// TLiteral<'z'>
// ]>