Store config in correct directory on macOS

[deanishe]

On macOS, glow creates the directory ~/Library/Preferences/glow/ and stores its config in there.

Applications should never write to ~/Library/Preferences. It's basically the macOS equivalent of the Windows registry, and shouldn't be modified directly:

Contains the user’s preferences. You should never create files in this directory yourself. To get or set preference values, you should always use the NSUserDefaults class or an equivalent system-provided interface.

https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/MacOSXDirectories/MacOSXDirectories.html

glow should store its config either under ~/Library/Application Support/ (the official directory for app-created files on macOS) or better yet under ~/.config/ (POSIX command-line programs are typically expected to follow POSIX conventions, not macOS ones).

[muesli]

Tracked upstream: muesli/go-app-paths#6

Not sure we'll change this at this point tho, if I'm honest.