build(deps-dev): Bump @ember/string from 3.1.1 to 4.0.0

Bumps [@ember/string](https://github.com/emberjs/ember-string) from 3.1.1 to 4.0.0.
- [Release notes](https://github.com/emberjs/ember-string/releases)
- [Changelog](https://github.com/emberjs/ember-string/blob/master/CHANGELOG.md)
- [Commits](https://github.com/emberjs/ember-string/commits)

---
updated-dependencies:
- dependency-name: "@ember/string"
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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