Provide a macOS installer

I see that the "ideas for improvement" in the front page contains:

* Add a Homebrew recipe for Bazelisk to make it easy to install on macOS.

... and I have quite a dislike for Homebrew. I'd suggest to provide a native macOS installer that places this tool in the correct location instead. It's actually quite easy to do so and avoids the need to install Homebrew (which some users may not have, like me, as I use pkgsrc).

Anyway, my main goal was to let you know that the packaging script I wrote for sandboxfs may help you get started: https://github.com/bazelbuild/sandboxfs/blob/master/admin/make-macos-pkg.sh . Was going to send you an email about this, but hey, I can also send this to you via a FR.