Add FileSystemObserver demo

[chrisdavidmills]

This PR adds a FileSystemObserver example to the repo, as per the conversation at mdn/content#38270 (comment) and follow-up at mdn/content#38270 (comment)

[bsmth]

Thanks!

[wbamberg]

On Firefox, https://github.com/chrisdavidmills/dom-examples/blob/6729dc1a0d1a050a64add4b364ff8fceb14dca69/filesystemobserver/random.js#L3 throws with:

Uncaught (in promise) TypeError: rootHandle.remove is not a function

...so the code at https://github.com/chrisdavidmills/dom-examples/blob/6729dc1a0d1a050a64add4b364ff8fceb14dca69/filesystemobserver/random.js#L11-L13 doesn't get to run.

[chrisdavidmills]

On Firefox, https://github.com/chrisdavidmills/dom-examples/blob/6729dc1a0d1a050a64add4b364ff8fceb14dca69/filesystemobserver/random.js#L3 throws with:

Uncaught (in promise) TypeError: rootHandle.remove is not a function

...so the code at https://github.com/chrisdavidmills/dom-examples/blob/6729dc1a0d1a050a64add4b364ff8fceb14dca69/filesystemobserver/random.js#L11-L13 doesn't get to run.

Yup. This is because the remove() method is not supported outside Chromium. But then again, neither is FileSystemObserver.

[wbamberg]

I've filed this as #298.