feat(behaviors): &key_repeat behavior + tests.

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

Closes: #853

docs/docs/behaviors/key-repeat.md

@@ -0,0 +1,38 @@
+---
+title: Key Repeat Behavior
+sidebar_label: Key Repeat
+---
+
+## Summary
+
+The key repeat behavior when triggered will send whatever keycode was last sent/triggered.
+
+### Behavior Binding
+
+- Reference: `&key_repeat`
+
+Example:
+
+```
+&key_repeat
+```
+
+### Configuration
+
+#### Usage Pages
+
+By default, the key repeat will only track the last pressed key from the HID "Key" usage page, and ignore events from other usages, e.g. Consumer page.
+
+If you'd rather have the repeat also capture and send Consumer page usages, you can update the existing behavior:
+
+```
+&key_repeat {
+    usage-pages = <HID_USAGE_KEY HID_USAGE_CONSUMER>;
+};
+
+/ {
+    keymap {
+        ...
+    };
+};
+```