Support macOS < 10.14 in repository releases

[chrissimpkins]

Related: #40

Users on macOS 10.13 and below cannot use the project macOS application package releases. It is necessary to either run from source (e.g., with a PyPI installation or cloned repository) or for the user to compile the package themselves (see https://slice-gui.netlify.app/docs/developer/#compile-app-distributions-by-platform). This is due to the backwards compatibility guarantee (or lack thereof) with the PyInstaller tool that is used to prepare the platform-specific distributions.

I think that I can address this by setting up a new, earlier macOS VM for the package build. Will require testing to confirm that the same build works on more recent versions of macOS.