roundingMode w/ -0

[FrankYFTang]

In http://userguide.icu-project.org/formatparse/numbers/rounding-modes
We see how each roundingMode return for some sample number
But in JavaScript we have -0 in additional to 0 :
What should -0 return when roundingMode: "up"? Should it be 0 or -1?
What should -0 return when roundingMode: "floor"? Should it be 0 or -1?

[sffc]

The idea with the signDisplay proposal was that we never change the sign of a number while rounding, except when we treat -0 and +0 as equivalent in exceptZero.

In this case, -0 is an integer, and it shouldn't round to anything, because it's an integer. -0 in = -0 out.

[sffc]

2021-02-11: Adopt #21 (comment).