build(deps-dev): bump web-ext from 5.5.0 to 6.0.0 in /addon (#837)

Bumps [web-ext](https://github.com/mozilla/web-ext) from 5.5.0 to 6.0.0.
- [Release notes](https://github.com/mozilla/web-ext/releases)
- [Commits](mozilla/web-ext@5.5.0...6.0.0)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]