Use floating-point numbers universally

[nex3]

Sass has never clearly specified its internal number format. Ruby Sass inherited Ruby's support for transparently switching to bignums when numbers got extremely large, and Dart Sass did the same thing initially until Dart dropped support for transparent bignums in 2.0.0. LibSass, on the other hand, has only ever used floating point numbers.

Although we could mandate the implementation of transparent bignums, this would add a substantial amount of implementation complexity and performance overhead since we'd have to do bounds-checking for every operation. Instead, I think just standardizing on double-width floating points makes sense, particularly because the compromises inherent in using floating point numbers everywhere are already well-understood by the web community thanks to JS.

Marking this as a bug because the behavior of large numbers is currently inconsistent in a problematic way.

[nex3]

Oops, looks like #2892 already exists.