hyprland: add option sourceFirst

Add the option sourceFirst to the hyprland module. When this option is
enabled source entries will be put near the top of the file, so that
the variables declared in other files can be used by the other
configuration entries.

Add "source" to the list of important prefixes when the former option
is enabled.

Resolves #4729

tests/modules/services/window-managers/hyprland/simple-config.conf

@@ -3,6 +3,7 @@ $mod=SUPER
 bezier=smoothOut, 0.36, 0, 0.66, -0.56
 bezier=smoothIn, 0.25, 1, 0.5, 1
 bezier=overshot, 0.4,0.8,0.2,1.2
+source=sourced.conf
 animations {
   animation=border, 1, 2, smoothIn
   animation=fade, 1, 4, smoothOut

tests/modules/services/window-managers/hyprland/simple-config.nix

@@ -8,6 +8,8 @@
     plugins =
       [ "/path/to/plugin1" (config.lib.test.mkStubPackage { name = "foo"; }) ];
     settings = {
+      source = [ "sourced.conf" ];
+
       decoration = {
         shadow_offset = "0 5";
         "col.shadow" = "rgba(00000099)";