feat(combos): initial implementation

closes #45

app/tests/combo/combos-and-holdtaps-0/native_posix.keymap

@@ -0,0 +1,47 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan-mock.h>
+
+&mt {
+	flavor = "hold-preferred";
+};
+
+/*
+This test fails if the order of event handlers for hold-taps
+and combos is wrong. Hold-taps need to process key position events 
+first so the decision to hold or tap can be made.
+*/
+/ {
+	combos {
+		compatible = "zmk,combos";
+
+		combo_two {
+			timeout-ms = <100>;
+			key-positions = <1 2>;
+			bindings = <&kp Y>;
+		};
+	};
+
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&mt LEFT_CONTROL A &kp B
+				&kp C &none
+			>;
+		};
+	};
+};
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(0,0,10) 
+		ZMK_MOCK_PRESS(0,1,10) 
+		ZMK_MOCK_PRESS(0,2,10) 
+		ZMK_MOCK_RELEASE(0,0,10) 
+		ZMK_MOCK_RELEASE(0,1,10) 
+		ZMK_MOCK_RELEASE(0,2,10)
+	>;
+};