Don't crash when using Infinity or NaN as a key in a map

[nex3]

Closes #3126
See sass/sass-spec#1556

[connorskees]

Would it make sense to instead deny both NaN and Infinity from being map keys? That map-get(((0/0): a), (0/0)) is equal to null may be surprising to some. As this was already a crash, it should not break any existing code relying on dart-sass. In fact, denying NaN and Infinity as map keys may prevent silent errors that result from the disparity in equality between libsass and dart-sass (see sass/libsass#3126).

[Awjin]

I'm ok with this workaround, and revisiting once floating point is officially defined in Sass.