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/dts/bindings/behaviors/zmk,behavior-hold-tap.yaml

@@ -30,3 +30,7 @@ properties:
       - "tap-preferred"
   retro-tap:
     type: boolean
+  hold-trigger-key-positions:
+    type: array
+    required: false
+    default: []