Dependencies haven't been updated in a while, and focus-trap, which depends on this module, uses Webpack now after focus-trap/focus-trap#124, so I'm also going to change the build system to Webpack.

I hope to address #51 while I'm at it.

Also along with focus-trap/focus-trap#124, and along with focus-trap/focus-trap-react#78, I'll be updating the ESLint and Prettier configuration to match. This way, all 3 repos now number the https://github.com/focus-trap "roof" will use the same stack and the same linting/formatting rules.

The project will move to Yarn since the other 2 repos use Yarn also.

Finally, I'll switch the CI to a GitHub Workflow, just like the other 2 repos are using now.