Behaviors: Add 'ignore-modifiers' option to sticky keys

To combine multiple sticky modifiers, the sticky keys must ignore
other (sticky) modifier keypresses.

This behavior is important for "callum-style mods", where all modifiers
are sticky mods.

Fixes #829

app/dts/behaviors/sticky_key.dtsi

@@ -12,6 +12,7 @@
 			#binding-cells = <1>;
 			release-after-ms = <1000>;
 			bindings = <&kp>;
+			ignore-modifiers;
 		};
 		/omit-if-no-ref/ sl: behavior_sticky_layer {
 			compatible = "zmk,behavior-sticky-key";