Fixes #34051 (closed).
This PR aims to resolve the Sass / deprecation issues from Dart Sass. One direction could've been to drop Libsass entirely, using the Dart Sass math module, but that'd effectively ruin any hope for support for Libsass-based projects. That matters not because we want to support something old and deprecated, but because there are dozens if not hundreds of projects that haven't updated to Dart Sass yet. Plus, it's easy enough for us to resolve on our own end without breaking things for others, so why not.
To make this happen, I've replaced the bulk of the division with multiplication, and then used a custom divide() function (thanks @alpadev!) to replace what I couldn't easily switch to multiplication.
Once we get this reviewed and merged, we'll be able to ship v5.0.2.
/cc @twbs/css-review