* Linux and Windows CI with GitHub Actions
* Add karma-edge-launcher
* Add edge configuration to karma.conf.js
* Support --browsers on the command line for karma tests
* Add macOS CI builds
* Add karma-safari-private-launcher
* Document browser specification for tests
Browserify isn't optimal bundling Chart.js because it adds too many internal wrappers, doesn't handle external/global dependencies and doesn't provide a way to generate ESM builds. Therefore, it seems the right choice to switch to rollup, so move all the build process in `rollup.config.js` and make Gulp to execute `rollup -c`.
We also had to switch to Terser instead of UglifyJS because this last one contains a breaking bug. Note that tests now use the exact same rollup config as our builds (the minified one) to ensure that the bundling and minification steps don't break anything. Finally, replace the `gulp watch` task by `gulp build --watch` to be consistent with the other `unittest` and `docs` watching syntax.
Setup a new Heroku app based on rauchg/slackin, using Slack legacy token from the Chart.js (chartjs.slack@...) user and reCAPTCHA from the same Google account.
Coverage data are now generated by running `gulp unittest` with the `--coverage` argument: unit tests are then executed a single time on Travis. The gulp `coverage` task has been removed and `karma.coverage.conf.ci.js` merged into `karma.conf.ci.js`.
Update documentation with gulp commands (and remove them from `README.md`) and remove unused `config.jshintrc` (oversight from #3256). Delete `thankyou.md` which has been merged into `README.md`.
