Normative: Delegate to NumberFormat to handle unit formattting

[FrankYFTang]

For the case of style "short", "long" and "narrow" and resolve the
plural issue and reduce unnecessary locale data handling.

This should also resolve the issue mentioned in #118

@ryzokuken @sffc @anba @gibson042 @michaelficarra @rbuckton @rkirsling -any opinion of moving toward this direction?

[ryzokuken]

@FrankYFTang thanks for this, I'd missed this simpler way of formatting the results which also avoids duplicating code as well as spec text.

[anba]

Probably worth mentioning this depends on tc39/ecma402#702 resp. tc39/ecma402#704

[ryzokuken]

@anba are we blocked on that right away or for Stage 4? Practically speaking, I suppose you'd need the change in order to implement this to begin with.

[anba]

are we blocked on that right away or for Stage 4?

I don't know. Are there any strict guidelines which describe cases like this? It's probably okay to merge this PR, as long as there's an <emu-note> to point out that this depends on the linked ECMA-402 issues. That way we don't risk that there's any confusion for other implementers of Intl.DurationFormat who didn't read this PR.

Practically speaking, I suppose you'd need the change in order to implement this to begin with.

Implementers can work around the unit option validation checks in the Intl.NumberFormat constructor if needed. But it's easier if the ECMA-402 issue is resolved first.

[ryzokuken]

@anba alright, thanks, that sounds good to me. I'll add a note and merge this.

[FrankYFTang]

Probably worth mentioning this depends on tc39/ecma402#702 resp. tc39/ecma402#704

Ah... I totally forgot about that dependency. Thanks for pointing that out.