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/behaviors.dtsi

@@ -15,3 +15,4 @@
 #include <behaviors/ext_power.dtsi>
 #include <behaviors/outputs.dtsi>
 #include <behaviors/caps_word.dtsi>
+#include <behaviors/key_repeat.dtsi>

app/dts/behaviors/key_repeat.dtsi

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <dt-bindings/zmk/keys.h>
+
+/ {
+	behaviors {
+		/omit-if-no-ref/ key_repeat: behavior_key_repeat {
+			compatible = "zmk,behavior-key-repeat";
+			label = "KEY_REPEAT";
+			#binding-cells = <0>;
+			usage-pages = <HID_USAGE_KEY>;
+		};
+	};
+};
+

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