Apps, refactor, clean up, remove librewolf

Refactors to use app.nix for easier extension.

Removes librewolf from host, as it makes more sense on the user every
time.

nixosModules/apps/shells/app.nix

@@ -0,0 +1,9 @@
+{ ... }:
+
+{
+  imports = [
+    ./fish
+    ./xonsh
+    ./zsh
+  ];
+}