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.

docs/sidebars.js

@@ -36,6 +36,7 @@ module.exports = {
       "behaviors/caps-word",
       "behaviors/key-repeat",
       "behaviors/sensor-rotate",
+      "behaviors/mouse-emulation",
       "behaviors/reset",
       "behaviors/bluetooth",
       "behaviors/outputs",