refactor: Add kscan sideband behavior driver

* Instead of gpio key behavior trigger, add new kscan driver that
  decorates/wraps a given kscan driver and will invoke basic system
  behavior assigned to a given row + column, without the need for keymap
  mapping in the matrix transform, bypassing keymaps entirely.

app/dts/bindings/kscan/zmk,kscan-sideband-behaviors.yaml

@@ -0,0 +1,29 @@
+# Copyright (c) 2023, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: |
+  kscan sideband behavior runner. Only basic system behavior should be used,
+  since no keymap processing occurs when using them.
+
+compatible: "zmk,kscan-sideband-behaviors"
+
+include: [kscan.yaml]
+
+properties:
+  kscan:
+    type: phandle
+    required: true
+
+child-binding:
+  description: "A sideband behavior tied to a row/column pair"
+
+  properties:
+    row:
+      type: int
+      required: true
+    column:
+      type: int
+      required: true
+    bindings:
+      type: phandle-array
+      required: true