Draft: Implement watching file paths via oslib.watch (for 0.12.x branch)

[arturaz]

Backport of #5068

The fs watching is disabled by default and can be disabled via --watch-via-fs-notify=true.

[arturaz]

Noticed this popping up while testing (note for self):

Exception in thread "Thread-535" java.nio.channels.ClosedChannelException
        at java.base/sun.nio.ch.FileChannelImpl.ensureOpen(FileChannelImpl.java:160)
        at java.base/sun.nio.ch.FileChannelImpl.write(FileChannelImpl.java:284)
        at java.base/sun.nio.ch.ChannelOutputStream.writeFully(ChannelOutputStream.java:68)
        at java.base/sun.nio.ch.ChannelOutputStream.write(ChannelOutputStream.java:105)
        at java.base/java.io.OutputStream.write(OutputStream.java:124)
        at mill.runner.Watching$.writeToWatchLog$1(Watching.scala:120)
        at mill.runner.Watching$.$anonfun$watchAndWait$15(Watching.scala:184)
        at mill.runner.Watching$.$anonfun$watchAndWait$15$adapted(Watching.scala:183)
        at os.watch.WatchServiceWatcher.run(WatchServiceWatcher.scala:152)
        at os.watch.package$$anon$1.run(package.scala:45)