Add "weekday" and "month" display names

[zbraniecki]

I couldn't find an issue for that, so please, feel free to dupe this against it if there is one.

In the process we trimmed down the scope of this API, and I'd like to verify that the current intention is to later add weekday and month display names.

[sffc]

Display names for month, weekday, day period, and date fields were temporarily removed in #63, because the data model with non-Gregorian calendars was unclear (#55). Now that the Temporal Calendar proposal is reaching Stage 3, it would be a good time to follow up on #55 and figure out the best way to make these display names scale to new calendar systems.

[FrankYFTang]

I put together a v2 repo on https://github.com/FrankYFTang/intl-displaynames-v2/ please move the future discussion there.

[FrankYFTang]

This feature is in the core of https://github.com/tc39/intl-displaynames-v2/ and this proposal itself is moved to Stage 4 in 2020 Sept TC39 meeting so I close this issue here. For future discussion, please move to https://github.com/tc39/intl-displaynames-v2/