docs: update pseudocode of `VRMC_springBone` and `VRMC_springBone_extended_collider`

[0b5vr]

This resolves #473

The use of direction and distance is already described in the VRMC_springBone spec This commit redirects the explanation in VRMC_springBone_extended_colliders to the VRMC_springBone one

I also added reference implementations for existing colliders of VRMC_springBone

[0b5vr]

nextTail については、もともとの擬似コードでも使用していますし、UniVRM・three-vrmのコードでも利用している命名なので、これで適切かなと思います。