feat: switch to using chokidar (#18)

Turbowatch was developed to leverage [Watchman](https://facebook.github.io/watchman/) as a superior backend for watching a large number of files. However, along the way, we discovered that Watchman does not support symbolic links (issue [#105](facebook/watchman#105 (comment))). Unfortunately, that makes Watchman unsuitable for projects that utilize linked dependencies (which is the direction in which the ecosystem is moving for dependency management in monorepos). As such, Watchman was replaced with [chokidar](https://www.npmjs.com/package/chokidar). We are hoping to provide Watchman as a backend in the future. Therefore, we made Turbowatch expressions syntax compatible with a subset of Watchman expressions.

BREAKING CHANGE:

* various miscellaneous expressions have been dropped
* debounce became part of the `watch` configuration
* change event no longer describes `exists`, `mtime` or `size` attributes of the file that changed