Integrate with Intl.DurationFormat

[ryzokuken]

#129 (comment) never really took Duration into account from what it seems, we should discuss this in a meeting.

[sffc]

We have an open proposal to add Intl.DurationFormat:

https:/younies/proposal-intl-duration-format

[ptomato]

Discussed in the March 19 meeting: We'll update the Intl.DurationFormat in our polyfill to match the one in the proposal as needed.

@ryzokuken Should this issue remain open to implement that, or is it more something that will be done on an ongoing basis as other issues are fixed?

[littledan]

I'm leaving out the Stage 3 milestone; I think the integration at both the spec and polyfill levels are more milestones for Intl.DurationFormat, and don't block Stage 3 or polyfill distribution for Temporal.