Improve the accessibility for the nav toggle button by adding a border
around the button when it is focussed on.
I've added a custom Tailwind CSS variant to do this using a custom
`focus-visible` variant in tailwind.config.js, and using the
https://www.npmjs.com/package/focus-visible polyfill.
- Updated npm dependencies
- Replaced `webpack.mix.js` with `webpack.config.js`
- Added `postcss.config.js`
- Removed the `src` directory and moved `css` and `js` to the root level
of the theme
- Renamed `dist` to `build` and updated in libraries file
References #41
