Bump scss-tokenizer and gulp-sass in /atomic-docs

Removes [scss-tokenizer](https://github.com/sasstools/scss-tokenizer). It's no longer used after updating ancestor dependency [gulp-sass](https://github.com/dlmanning/gulp-sass). These dependencies need to be updated together.


Removes `scss-tokenizer`

Updates `gulp-sass` from 4.1.0 to 5.1.0
- [Release notes](https://github.com/dlmanning/gulp-sass/releases)
- [Changelog](https://github.com/dlmanning/gulp-sass/blob/master/CHANGELOG.md)
- [Commits](dlmanning/gulp-sass@v4.1.0...v5.1.0)

---
updated-dependencies:
- dependency-name: scss-tokenizer
  dependency-type: indirect
- dependency-name: gulp-sass
  dependency-type: direct:development
...

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