feat(mouse): Initial mouse keys support.

* Add HID report/descriptor for a new report with mouse buttons,
  and x/y/wheel deltas.
* New mouse key press behavior for press/release of mouse keys.
* Add constants for HID main item values (e.g. data/array/absolute)
* Define and use constants for our HID report IDs.

app/include/dt-bindings/zmk/hid_usage_pages.h

@@ -26,6 +26,7 @@
 #define HID_USAGE_GDV (0x06)            // Generic Device Controls
 #define HID_USAGE_KEY (0x07)            // Keyboard/Keypad
 #define HID_USAGE_LED (0x08)            // LED
+#define HID_USAGE_BUTTON (0x09)         // Button
 #define HID_USAGE_TELEPHONY (0x0B)      // Telephony Device
 #define HID_USAGE_CONSUMER (0x0C)       // Consumer
 #define HID_USAGE_DIGITIZERS (0x0D)     // Digitizers

app/include/dt-bindings/zmk/mouse.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+#pragma once
+
+#include <zephyr/dt-bindings/dt-util.h>
+
+/* Mouse press behavior */
+/* Left click */
+#define MB1 BIT(0)
+#define LCLK (MB1)
+
+/* Right click */
+#define MB2 BIT(1)
+#define RCLK (MB2)
+
+/* Middle click */
+#define MB3 BIT(2)
+#define MCLK (MB3)
+
+#define MB4 BIT(3)
+#define MB5 BIT(4)