Add cmd/fsnotify

[arp242]

This adds a little example that can be run:

% go run ./cmd/fsnotify **/*
12:76:11.4710 watching; press ^C to exit
12:76:13.6549   1 REMOVE                "cmd/fsnotify/4913"
12:76:13.6550   2 RENAME                "cmd/fsnotify/main.go"
12:76:13.6550   3 RENAME                "cmd/fsnotify/main.go"
12:76:13.6612   4 CREATE                "cmd/fsnotify/main.go"
12:76:13.6612   5 WRITE      (modified) "cmd/fsnotify/main.go"
12:76:13.6614   6 WRITE      (modified) "cmd/fsnotify/main.go"
12:76:13.6648   7 CHMOD                 "cmd/fsnotify/main.go"
12:76:15.2919   8 CREATE                "cmd/fsnotify/4913"
12:76:15.2919   9 REMOVE                "cmd/fsnotify/4913"
12:76:15.2922  10 CHMOD                 "cmd/fsnotify/main.go"
12:76:15.2923  11 REMOVE                "cmd/fsnotify/main.go"
12:76:15.2925  12 RENAME                "cmd/fsnotify/main.go"
12:76:15.2986  13 CREATE                "cmd/fsnotify/main.go"
12:76:15.2986  14 WRITE      (modified) "cmd/fsnotify/main.go"
12:76:15.2988  15 WRITE      (modified) "cmd/fsnotify/main.go"
12:76:15.3024  16 CHMOD                 "cmd/fsnotify/main.go"
^C

It's mostly the same as the example in the README, except that it takes
paths from the commandline and aligns things a bit nicer.

This is useful for documentation and experimentation. For example, while
reviewing some PRs I found it useful to quickly get an overview of "what
does fsnotify do now, and what does it do with this patch?"

[jolheiser]

Just as my 2c I feel like log would probably suffice over line and/or fatal, and cmd seems odd for an example vs examples or contrib since it's more of a helper utility.

But it seems like a handy way to throw together a quick test overall.

[arp242]

I mostly didn't use log because I wanted only the time with ms, and not the full date, which doesn't strike me as useful here and it saves 11 characters 😅 Maybe log can be configured (I never really used it), but just fmt seemed simpler.

I typically like putting these things in cmd as it's a "runnable program" so it seems to fit; but can put it somewhere else too; I don't care that much. I'd also like to expand this tool a little bit to add some more things like printing things like system information, volume information, etc. which might be useful for bug reports (but that's something for another day).