RFE: teach sway -C to obey include directives.

[james-lawrence]

given a sway config that has 2 files:

# ~/.config/sway/config
set $mod Mod4

include config.d/*

# ~/.config/sway/config.d/config2
bindsym $mod+question echo "help!"

running sway -C -c ~/.config/sway/config doesn't validate the entire configuration just the exact file.
running sway -C -c ~/.config/sway/config.d/config2 will fail because its relying on state from the parent configuration.

this means multi file configurations can't be validated.