Define well-known locations

[kenchris]

In response to w3ctag/design-reviews#390

Other platforms have default locations that can be reused, like

Android:
https://developer.android.com/reference/android/os/Environment#DIRECTORY_ALARMS

Linux:
https://wiki.archlinux.org/index.php/XDG_user_directories
https://cgit.freedesktop.org/xdg/xdg-user-dirs/tree/user-dirs.defaults

[christianliebel]

Windows:
https://docs.microsoft.com/de-de/windows/desktop/api/shlobj_core/nf-shlobj_core-shgetknownfolderpath

[mkruisselbrink]

Closing this. #144 will need something like this, and other API surfaces might too, but with getSystemDirectory no longer currently specified this isn't currently needed.