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/zmk/mouse.h

@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <dt-bindings/zmk/mouse.h>
+
+typedef uint8_t zmk_mouse_button_flags_t;
+typedef uint16_t zmk_mouse_button_t;