feat: use watchman_client

[nekowinston]

Closes #14.
I think I implemented the changes correctly, but I'm not 100% confident yet, thus marking it as WIP.
Just wanted to get the ball rolling on the issue.

[nekowinston]

I've been comparing the two outputs of the master branch and the PR branch with

export CLOCK=$(watchman clock . | jq -r .clock)
diff <(~/Code/rs-git-fsmonitor/target/release/rs-git-fsmonitor 2 "$CLOCK" | xxd) <(/nix/store/.../bin/rs-git-fsmonitor 2 "$CLOCK" | xxd)

and I'm pretty sure the output is correct.

Some notes:
If I understand resolve_root correctly, it uses the watch-project subcommand under the hood instead of the (deprecated) watch subcommand; which might lead to watchman watch-list differing to the current master branch.

Also, Connector::new will still spawn the underlying watchman CLI to figure out which watchman socket to use – I see a way better speedup compared to the master branch if I set WATCHMAN_SOCK as an environment variable.