feat(browsers): add firefox

- add nur
- set firefox as main browser
- move browsers into separate modules

home/browsers/default.nix

@@ -0,0 +1,9 @@
+{ config, pkgs, ... }:
+
+{
+  imports = [
+    ./firefox.nix
+    ./brave.nix
+    ./chromium.nix
+  ];
+}