refactor: Move nix outside of .config dir

Technically not a config but a compose file, so I think it's better to
have outside of `.config` dir

nix/modules/home-manager/floorp.nix

@@ -0,0 +1,13 @@
+{ pkgs, config, vars, ... }:
+
+{
+  programs.floorp = {
+    enable = true;
+    # REF: https://github.com/nix-community/home-manager/blob/342a1d682386d3a1d74f9555cb327f2f311dda6e/modules/programs/firefox/mkFirefoxModule.nix#L264
+    package = null;  # we only want the config
+    profiles.${config.home.username} = {
+      id = 0;
+      isDefault = true;
+    };
+  };
+}