waybar: merge multiple style definitions (#4038)

This is backwards compatible and allows for more flexibility(eg.
allows for defining custom waybar modules in separate nixos modules,
before merging them together)

Co-authored-by: mae <git@badat.dev>

modules/programs/waybar.nix

@@ -212,7 +212,7 @@ in {
     };
 
     style = mkOption {
-      type = nullOr (either path str);
+      type = nullOr (either path lines);
       default = null;
       description = ''
         CSS style of the bar.