expose flatten as accessor pair on Array.prototype

[michaelficarra]

As an alternative to #56, @bakkot suggested we take the self-redefining accessor pair approach. This is a quick attempt at what that spec text might look like. I warn you, I was very tired while writing this.

_{Also, personally, I still prefer smoosh.}

[michaelficarra]

Resolved by https://github.com/tc39/proposal-flatMap/commit/093eacc7fe0906e70f7626bf6c7d6e9dfc53cce9.