Refactor global bindings, implement mod-tap.

* Use extra comptible = "zmk,behavior-global" to add
   behaviors to global bindings for event notification.
* Implement mod-tap, as a keymap binding and global one
  to skip tap if other keycode pressed while held.

app/dts/behaviors/mod_tap.dtsi

@@ -0,0 +1,9 @@
+/ {
+	behaviors {
+		mt: behavior_mod_tap {
+			compatible = "zmk,behavior-mod-tap", "zmk,behavior-global";
+			label = "MOD_TAP";
+			#binding-cells = <2>;
+		};
+	};
+};