Add Watcher implementations

[devongovett]

This adds subscribe and unsubscribe methods to watch for changes to a directory. There are several backends:

• FSEvents on macOS
• Inotify on Linux
• ReadDirectoryChangesW on Windows
• watchman if available

Unlike nsfw, which starts up a thread per watched directory, fschanges starts up a single thread per backend and adds multiple watchers to it. Once all watchers have been closed, the backend thread is also closed.

Events are also denounced. Rather than spamming the JS thread when file changes come in, we queue up changes and emit them when things settle down. This is very useful for e.g. npm installs, git checkouts, or other large changes where we end up calling into JS only once with all of the events rather than separately for each one.

Another advantage to having the watchers be part of this package is that we can keep our in-memory snapshot up to date incrementally instead of potentially traversing the entire filesystem again on exit. This should make Windows and Linux a bit faster.

[mischnic]

subscribe and getEventsSince returns relative paths with watchman and absolute paths with fsevents

fschanges (watcher *)$ node index.js
[ { path: 'yarn.lock', type: 'update' } ]
[ { path: 'yarn.lock', type: 'update' } ]
[ { path: 'index.js', type: 'update' } ]
^C
(watcher *)$ node index.js
[ { path:
     '/Users/niklas/.../parcel/fschanges/yarn.lock',
    type: 'update' } ]

[devongovett]

yeah I was planning on writing some tests to ensure consistency next.

[devongovett]

🎉