Update eslint, fix linting errors and integrate in CI

This repo has an .eslintrc, added in #244: https://github.com/mdn/webextensions-examples/blob/main/.eslintrc.json

... but it is stuck in 2017 and it doesn't appear to be enforced (evidenced by the plenty of failures when I run the following from the root of the repo:

npm install
npm test   # this is equivalent to: ./node_modules/.bin/eslintrc .

eslint used to be run in CI, but for some unknown reason it was dropped in #259.
We should re-enable CI. Move to Github Actions instead of Travis CI, because Travis CI is no longer free and most mdn/Mozilla projects have switched already.

Task list:

• Update eslint to latest version, by bumping: https://github.com/mdn/webextensions-examples/blob/9433f842d6295db366c399df752332d78a9cc859/package.json#L7
• Update .eslintrc to support modern syntax and APIs.
• Resolve or suppress linter warnings when npm test is run.
• Replace the Travis with Github actions: https://docs.github.com/en/actions/migrating-to-github-actions/manual-migrations/migrating-from-travis-ci-to-github-actions

Review package.json and .eslintrc.json, then run npm install and npm test to inventory the current lint failures. Inspect the existing Travis CI setup before planning its GitHub Actions replacement. Done means modern ESLint support, resolved or intentionally suppressed warnings, passing npm test, and linting enforced in CI.