Add support for fs.watch().

[dpryden]

This support currently only works for Node v10 and newer, since the
fs_event_wrap native binding is lazy-loaded and thus can be pre-patched
before first use.

To make this robust enough to actually use, it may be necessary to also
add in monkeypatching of the fs.watch endpoint.

The tests are incomplete; mainly just enough to demonstrate that this
works as a proof of concept.

[tschaub]

Thanks for the work on this, @dpryden. I didn't intend to close this pull request – forgot that that happens automatically when changing default branches. We can still use these commits if someone else wants to take up this work. Or feel free to open a new pull request (you should be able to git pull --rebase upstream main && git push --force origin fs-watch depending on how you have named your remotes).

[dpryden]

OK, I rebased (and fixed a minor merge conflict) and created #308 as a replacement. Thanks for reminding me about this PR @tschaub !

I no longer have a pressing need for this change but I'm happy to help move it forward. However, realistically I think it might be easiest to wait until mock-fs is willing to drop support for old versions of Node... and I have no idea when that might happen, or if it's worth investing more effort in this now.