build(deps-dev): bump eslint-plugin-mozilla in /addon

Bumps [eslint-plugin-mozilla](http://firefox-source-docs.mozilla.org/tools/lint/linters/eslint-plugin-mozilla.html) from 3.7.5 to 4.2.1.

---
updated-dependencies:
- dependency-name: eslint-plugin-mozilla
  dependency-version: 4.2.1
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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Author: dependabot[bot]