Add enum.Builder

[orsinium]

Add enum.Builder which can be used to construct a new enum:

type Color enum.Member[string]

var (
  b      = enum.NewBuilder[string, Color]()
  Red    = b.Add(Color{"red"})
  Green  = b.Add(Color{"green"})
  Blue   = b.Add(Color{"blue"})
  Colors = b.Enum()
)

The goal is to prevent situations when the enum may evolve as the project grows and people might forget to add new members to the enum.

I'll leave the PR open so you folks can leave your comments. Do you find it useful? Do you have ideas on how to make it better?

[orsinium]

Let's do this!