Bugfix: Display negative sign on leading numeric-style zeroes

[ben-allen]

Bugfix: FormatNumericUnits previously incorrectly dropped the negative sign when:

• A negative duration is being formatted
• The first displayed unit is a "numeric" or "2-digit" hour or minute value
• The value of that hour or minute is 0

This adds tests for these conditions to FormatNumericUnits.

fixes https://github.com/tc39/proposal-intl-duration-format/issues/204

[ben-allen]

I'm tentatively thinking this is a bugfix rather than a normative change, though I've requested time on the plenary agenda in case others tell me it's normative. thoughts? @ryzokuken @gibson042

[ben-allen]

Achieved consensus at July 2024 plenary