Add Arch Linux PKGBUILD

[gjabell]

Is your feature request related to a problem? Please describe.
Baker already has OS packages for MacOS, Windows, and Debian/Ubuntu, but is lacking them for other Linux distros.

Describe the solution you'd like
Add an Arch Linux PKGBUILD, which describes how the system should install / update Baker as a system-wide package.

Describe alternatives you've considered
At the moment, installing Baker on Arch Linux requires installing from source, which doesn't allow updating through the system package manager.

Additional context
PKGBUILDs can also be submitted to the Arch Linux User Repository (AUR), which makes it easy for users to download (and update) the package directly through their package manager, instead of manually running the makepkg command.