Support for KHR_animation_pointer

While Gestaltor, Babylon and Needle Engine (based on three.js) have already implemented _KHR_animation_pointer_, the validator currently doesn't support it. That leads to thousands and sometimes hundreds of thousands of errors for files that work fine:

The errors are many times
```json
{
"code": "VALUE_NOT_IN_LIST",
"message": "Invalid value 'pointer'. Valid values are ('translation', 'rotation', 'scale', 'weights').",
"severity": 1,
"pointer": "/animations/0/channels/2/target/path"
},
```

and
```json
{
"code": "ANIMATION_DUPLICATE_TARGETS",
"message": "Animation channel has the same target as channel 353.",
"severity": 0,
"pointer": "/animations/2/channels/9/target"
},
```

Are there plans / what are the plans for supporting _KHR_animation_pointer_ to ensure files are correctly set up? I understand that this is kind of a chicken-and-egg situation between the extension not being ratified (yet) and the need to produce correct files for it, but wanted to ask nonetheless :)

Thanks!

Start by reviewing the validator's handling of animation channel target paths and the KHR_animation_pointer extension behavior described in this issue. Reproduce the reported VALUE_NOT_IN_LIST and ANIMATION_DUPLICATE_TARGETS diagnostics with a file using the extension; done means valid files no longer produce those incorrect errors.

Written by the indexing model from the issue text.