Add the FileSystemObserver API

[nathanmemmott]

Add the FileSystemObserver API

Defines the FileSystemObserver API, an API to observe file system change events.

See #123 and WICG/file-system-access#72

• [x] At least two implementers are interested (and none opposed):
  • Chrome
  • Firefox
• [ ] Tests are written and can be reviewed and commented upon at:
  • …
• [ ] Implementation bugs are filed:
  • Chromium: https://crbug.com/320560741
  • Gecko: …
  • WebKit: …
• [ ] MDN issue is filed: …
• [x] The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

---

Preview | Diff

[nathanmemmott]

The errors causing the build to fail were not introduced by this PR.