Investigate using union type for `key`

[Dylan Irlbeck]

In src/re/Types.re, we could try and use a "union" type for key, instead of rewriting the code for the style and color type. For reference, see these articles:

• https://reasonml.org/blog/union-types-in-bucklescript
• https://dev.to/yawaramin/emulating-typescript-union-types-with-reasonml-3ea0