feat(es2018): add definitions for new array methods (flatten, flatMap)

[benbraou]

Array.prototype.flatMap and Array.prototype.flatten (https://github.com/tc39/proposal-flatMap) are now at stage 3

This issue is created to handle the addition of the corresponding definitions.

This has been suggested by @mhegazy #20385 (comment)

[benbraou]

I will provide a PR for this

[benbraou]

Array.prototype.flatten and Array.prototype.flatMap will be marked for the moment as esnext and not es2018.
After TC39 January meeting, it will be clearer whether this will make it to es2018