Support 2D skeletal animation

[robothot]

Support skeleton, which can be easily applied in drawing people

Here is some information about the 2d skeleton

• https://github.com/urraka/skel2d
• https://docs.godotengine.org/en/stable/tutorials/animation/2d_skeletons.html
• https://unity.com/features/2danimation
• https://nccastaff.bmth.ac.uk/pjunjun/publications/2d%20paper.pdf

[robothot]

@asturur @ShaMan123 I believe this is a great idea, what do you guys think about it?