feat(endpoints): add preferred endpoint setting

Added a new setting to remember the user's preferred endpoint. When both USB and
BLE are connected, the preferred endpoint will be used.

Added a new behavior to control this setting. It supports commands:

    &end END_USB - Prefer USB output
    &end END_BLE - Prefer BLE output
    &end END_TOG - Toggle between USB and BLE

docs/sidebars.js

@@ -22,6 +22,7 @@ module.exports = {
       "behavior/mod-tap",
       "behavior/reset",
       "behavior/bluetooth",
+      "behavior/endpoints",
       "behavior/lighting",
       "behavior/power",
     ],