Rotate the upper body then head

[vincentfretin]

Currently I rotate only the head when rotating the avatar.
Move a bit the upper body (shoulders and/or neck) then head when rotating the avatar, a bit like the avatar in Meta home space.
There is some code related to that in https://github.com/msub2/avatars/blob/master/src/vrarmik/ShoulderPoser.js but I couldn't make much sense of it with the valid avatars.