build(deps-dev): bump web-ext from 6.3.0 to 6.6.0 in /addon (#1217)

Bumps [web-ext](https://github.com/mozilla/web-ext) from 6.3.0 to 6.6.0.
- [Release notes](https://github.com/mozilla/web-ext/releases)
- [Commits](mozilla/web-ext@6.3.0...6.6.0)

---
updated-dependencies:
- dependency-name: web-ext
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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