feat(behaviors) Required keys for tap-hold behaviors

* Add optional `hold-trigger-key-positions` hold-tap configuration
* Leverage configuration for decision making around when to trigger
  hold decision in hold-taps.
* Add docs for new configuration.
* Tests for the new config/decision logic.

app/tests/hold-tap/balanced/7-positional/behavior_keymap.dtsi

@@ -0,0 +1,29 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+
+/ {
+	behaviors {
+		ht_bal: behavior_hold_tap_balanced {
+			compatible = "zmk,behavior-hold-tap";
+			label = "HOLD_TAP_BALANCED";
+			#binding-cells = <2>;
+			flavor = "balanced";
+			tapping-term-ms = <300>;
+			quick-tap-ms = <200>;
+			bindings = <&kp>, <&kp>;
+            hold-trigger-key-positions = <2>;
+		};
+	};
+
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&ht_bal LEFT_SHIFT F      &ht_bal LEFT_CONTROL J
+				&kp D      &kp E>;
+		};
+	};
+};