Bump scss-tokenizer and @rails/webpacker

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


Removes `scss-tokenizer`

Updates `@rails/webpacker` from 4.0.6 to 5.4.3
- [Release notes](https://github.com/rails/webpacker/releases)
- [Changelog](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
- [Commits](rails/webpacker@v4.0.6...v5.4.3)

---
updated-dependencies:
- dependency-name: scss-tokenizer
  dependency-type: indirect
- dependency-name: "@rails/webpacker"
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>