implementation-defined behavior for Intl.DisplayNames options

[leobalter]

The review from #502 got too verbose, I'd like to bring back a pending feedback from @anba:

https://tc39.es/ecma402/#conformance lists options which are allowed to have implementation-defined behaviour. For Intl.DisplayNames, the "style" and "type" options should probably be listed there. I'm not yet sure if it makes sense to allow implementation-defined behaviour for the "fallback" option, because I don't exactly know what other kind of fallback should happen here.

Originally posted by @leobalter in #502 (comment)