Modernize Sass import rules by using `@use` and `@forward`

## Problem
`make html` produced a bunch of deprecation warnings.
```javascript
WARNING in ./src/crate/theme/rtd/crate/static/vendor/furo/styles/furo.sass (./node_modules/css-loader/dist/cjs.js??ruleSet[1].rules[0].use[1]!./node_modules/postcss-loader/dist/cjs.js??ruleSet[1].rules[0].use[2]!./node_modules/sass-loader/dist/cjs.js??ruleSet[1].rules[0].use[3]!./src/crate/theme/rtd/crate/static/vendor/furo/styles/furo.sass)
Module Warning (from ./node_modules/sass-loader/dist/cjs.js):
Deprecation Warning on line 2, column 8 of file:///Users/bmunkholm/code/c/docs/crate-docs-theme/src/crate/theme/rtd/crate/static/vendor/furo/styles/furo.sass:2:8:
```

## Evaluation
Sass `@import` rules are deprecated and will be removed in Dart Sass 3.0.0.

More info and automated migrator: https://sass-lang.com/d/import
2 | https://github.com/import "variables"

## Poke
An attempt to fix that was done in #657 but reverted again in #672 due to #668.

## Next
It would be nice to figure out how to get rid of those warnings properly as it hides "real" warnings as we get used to ignore warnings when building.