Provide graphical / one-click installers

Not happening on Linux, though. But there's certain value in allowing an initial install of RGBDS to be made easier: Windows users may be too timid to touch the command line yet beyond batch files, and thus would have trouble with a package manager.

We do have a simple solution for creating such an installer: CPack provides a "Inno Setup" packager, which could provide an 80/20 output. (I wouldn't put much more effort than this into it.)

macOS has `.pkg` files as an option, but I don't know much about those.