Add bonemerging and model bone parenting to holos

[Denneisk]

1. Allows holos to bonemerge to their parents
   • This allows players to bonemerge, for example, a hat to their head
   • holoBonemerge(number, number) - Simple switch function with second argument controlling state. Does not operate if holo is not parented.
2. Allows holos to parent to model bones (followbone)
   • This is roughly equivalent to bonemerging, but less convenient
   • Lets users do PAC3 stuff with holos, basically
   • holoParentBone(number, entity, number)
   • holoParentBone(number, number, number) - Direct parent to another holo
3. Adds some model bone accessor functions to get model bone information via E2
   • These were added so users do not need to rely on external methods to comfortably use holo bone parenting
   • entity:getModelBoneCount()
   • entity:getModelBoneIndex(string)
   • entity:getModelBoneName(number)
   • entity:getModelBones()
4. Added holoParentAttachment(number, number, string) as a direct holo-to-holo attachment parent
   • This is superfluous and probably is handled better by using the Holo entity directly, but for some reason I thought the functional symmetry with holoParent was especially important here.