refactor(oneOf validator): Refactor oneOf validator

[zakhenry]

... to be less tied to ngx-subform implementation. Restructured errors for oneOf behaviour to be explicitly named rather than implicit by array index location

[zakhenry]

This is still WIP, opened for discussion

[zakhenry]

One other thought I had - is this even a good idea to have in this package?

I’m wondering if we should create a NgxFormGroupValidators package? It really feels like a distinct package and I can already think of a few cases we would want already:

• oneOf (polymorphism)
• equals (password creation)
• greaterThan (min/max fields)
• ordered (drawing tool profiles)

[maxime1992]

discussed offline with @zakhenry:

• can't really do a oneOf pattern without ngx sub form anyway :shrug:
• easy to make a breaking change later on if we move that to a separate repo

So all good to have that on ngx sub form :+1: