Extends current enum to add new cases #5

Enums in Python allow a restricted form of subclassing, where you can define methods on a base (or mix-in) class. You cannot, however, extend a base enum to add new enum members.

In C#, you cannot extend an enum, but you can use extension methods to add related methods.

In TS, you cannot extend an enum via subclassing, but we do support declaration merging as long as the enums are in the same file (which I am not advocating for this proposal).

I don't believe subclassing enums should be considered for this proposal.

Briefly: I'm in favor of enabling some kind of re-use of enum values between enums, but I think it sets up the wrong expectations to present it as "extending" the enum, and to use the extends keyword.

Here's why: extends for classes and interfaces means two things in practice:

1. You get all the relevant fields/properties/methods/values from the base type (what we want)
2. You get to be considered a subtype/derived type of the base type (very wrong in the case of enums)

The README already says that the enum is "non-extensible". This is the right default; sum types like enums should be non-extensible, because that's how you get value from matching/destructuring them. The second connotation of extends undermines that.