Create `Unions.Add` function

[ThomasAribart]

Hello and thanks for this awesome lib 🙌

Similar to Tuples.Append, it'd be nice to have a Unions.Add function to include new elements to a union:

type Test = Pipe<1, [Unions.Add<2>]>
// => 1 | 2

[gvergnaud]

Feel free to open a PR if you want to add it :)