Replace division with non-deprecated syntax

[nogweii]

This fixes a warning about Dart Sass 2.0.0 removing support for literal division using just the / symbol.

Closes #220.

[nogweii]

This isn't necessary for v4 as you've avoided using division entirely there. It'd be nice to have a minor version bump including this change, but in the mean time others can include this fix in their package.json like so:

{
  "dependencies": {
    "keyrune": "git+https://github.com/nogweii/keyrune.git#5737f2466f4af4a9bf383c06385cee5304ae7a98"
  }
}