feat(behaviors): &key_repeat behavior + tests.

* Add new `&key_repeat` behavior that captures and re-sends
  the most recently triggered keycode.

Closes: #853

app/dts/bindings/behaviors/zmk,behavior-key-repeat.yaml

@@ -0,0 +1,13 @@
+# Copyright (c) 2021 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Key repeat behavior
+
+compatible: "zmk,behavior-key-repeat"
+
+include: zero_param.yaml
+
+properties:
+  usage-pages:
+    type: array
+    required: true