feat(behaviors): Add caps word (&caps_word).

* Add new `&caps_word` behavior that acts like caps lock, but
  releases automatically when any "break" keycode is pressed.

app/dts/behaviors.dtsi

@@ -14,3 +14,4 @@
 #include <behaviors/bluetooth.dtsi>
 #include <behaviors/ext_power.dtsi>
 #include <behaviors/outputs.dtsi>
+#include <behaviors/caps_word.dtsi>

app/dts/behaviors/caps_word.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/ caps_word: behavior_caps_word {
+			compatible = "zmk,behavior-caps-word";
+			label = "CAPS_WORD";
+			#binding-cells = <0>;
+			continue-list = <UNDERSCORE>;
+		};
+	};
+};
+

app/dts/bindings/behaviors/zmk,behavior-caps-word.yaml

@@ -0,0 +1,15 @@
+# Copyright (c) 2021 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: Caps word behavior
+
+compatible: "zmk,behavior-caps-word"
+
+include: zero_param.yaml
+
+properties:
+  continue-list:
+    type: array
+    required: true
+  mods:
+    type: int