Include chokidar plugin

See [Hot-Reloading using Webpack with PHP File Changes](https://dev.hmn.md/2018/11/27/hot-reloading-using-webpack-with-php-file-changes/).

It'd be nice to bundle [chokidar](https://href.li/?https://github.com/paulmillr/chokidar) as a dependency here and provide some basic defaults for using it to reload browsers on any changes to PHP files. I'm undecided whether it makes sense to include it in the default development preset, but even if it's not included by default, it'd be nice to include a function that can be passed into `devserver.before` to enable the PHP watch functionality.