modules: move style to nix/style

This is temporary, as we're going to get rid of that awkward style
module that was meant to be shared, but ended up being too specific to
my use-case. We just put it in a nix directory for now, as we will with
everything else. This will be refactored later to be mixed with the
relevant program's configuration.

modules/default.nix

@@ -1,6 +1,6 @@
 {
   imports = [
-    ./style
     ./options
+    ./nix
   ];
 }

modules/nix/default.nix

@@ -0,0 +1,3 @@
+{
+  imports = [./style];
+}