Migrate Homebrew Cask to a separate repository

[AlternateRT]

Right now, the Homebrew Cask for Zen is being hosted directly inside the zen-browser/desktop repository; this is not ideal. It should be moved to a separate repository, ideally zen-browser/homebrew-browser to keep things mostly the same.

Rationale

The way things currently works means that whenever anyone runs:

brew tap zen-browser/browser https://github.com/zen-browser/desktop.git

they are unwittingly cloning the entire zen-browser/desktop repository. This is not how Homebrew is supposed to work.

Third-party repositories for Homebrew Casks (or simply Taps) are only meant to host the cask you want Homebrew to track, update, and install for you (in the case for Zen, that cask is only zen-browser).

In addition, if the Casks/zen-browser.rb file were to be moved to a dedicated repository, everyone would benefit of being able to use the one-argument form of brew tap. So the command from above would instead become:

brew tap zen-browser/browser