Added typescript support for animations

[HugoMcPhee]

Hi thanks for this library and the latest typescript updates in 2.0,

This PR adds typescript support for animations tested with:

• Fox with multiple animations from https://github.com/KhronosGroup/glTF-Sample-Models/blob/master/2.0/Fox/glTF-Binary/Fox.glb
• and a custom blender exported glb

The main changes are:

• root bones are added to the GLTFResult type

  type GLTFResult = GLTF & {
  nodes: {
  fox: THREE.SkinnedMesh
  _rootJoint: THREE.Bone
  }
  materials: {
  fox_material: THREE.MeshStandardMaterial
  }
  }

  which removes type errors for <primitive object={nodes._rootJoint} />

• an ActionName and GLTFActions type are added

  type ActionName = 'Survey' | 'Walk' | 'Run'
  type GLTFActions = Record<ActionName, THREE.AnimationAction>

• default values/types are given to actions and mixer

  const actions = useRef(null as null | GLTFActions)
  const [mixer] = useState(() => new THREE.AnimationMixer(nodes.fox))

  the mixer uses the first skinned mesh it finds, otherwise null

Note: the "npx lint-staged" isn't required, and was due to having trouble commiting

[drcmda]

nice, thanks!