home: keyboard: extract from X module

This deserves to be its own standalone module, as I would want to use it
in both X and Wayland, etc...

modules/home/keyboard/default.nix

@@ -0,0 +1,16 @@
+{ config, lib, ... }:
+let
+  cfg = config.my.home.keyboard;
+in
+{
+  options.my.home.keyboard = with lib; {
+    enable = my.mkDisableOption "keyboard configuration";
+  };
+
+  config = lib.mkIf cfg.enable {
+    home.keyboard = {
+      layout = "fr";
+      variant = "us";
+    };
+  };
+}