read default keyboard config from XKB_DEFAULT_* environment variables

[quantenzitrone]

Description

Other wayland compositors read the keyboard config from these variables: XKB_DEFAULT_RULES XKB_DEFAULT_MODEL XKB_DEFAULT_LAYOUT XKB_DEFAULT_VARIANT XKB_DEFAULT_OPTIONS Cage does it: https://github.com/cage-kiosk/cage/wiki/Configuration#xkb-environment-variables Sway does it, tho I couldn't find any docs about it or the code that reads these variables. Maybe it's in wlroots. It would be cool to have this as a way to (to some degree) wayland compositor independently set a keyboard layout.