feat(behaviors): Add soft off behavior.

* New soft-off behavior that can be used to force the device
  into  soft-off state with only certain configured wakeup
  devices.

app/dts/bindings/behaviors/zmk,behavior-soft-off.yaml

@@ -0,0 +1,14 @@
+# Copyright (c) 2023 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Soft-Off Behavior
+
+compatible: "zmk,behavior-soft-off"
+
+include: zero_param.yaml
+
+properties:
+  hold-time-ms:
+    type: int
+    required: false
+    description: Number of milliseconds the behavior must be held before releasing will actually trigger a soft-off.