DurationFormat: `days` should support `numeric` format too

[justingrant]

I've seen NASA-style countdown timers that include days as a top-level unit, with display like this:T-3:23:58:59 which means almost 4 days before launch time. The days use the same separator as between hours and minutes.

Seems like we may want to support the days: 'numeric' style to support this case.

Here's one example I found online.

[sffc]

The issue here would be the lack of CLDR data.

[ryzokuken]

The current proposal doesn't include support for numeric days, but it can be easily added later, once CLDR data is available. @justingrant can this be closed?

[romulocintra]

Actions :

• [ ] Open CLDR issue to support this feature @sffc