firstPerson: implement curve and document xRange

[lyuma]

The firstPerson object has information about the input and output ranges that might be useful for applications doing manual eye tracking.

Additionally, it's possible to create custom curves. using json["extensions"]["vrm"]["firstPerson"]["lookAtHorizontalInner"]["curve"] - it would be good to use this for animation interpolation if available.

Document the fact that the animation track for eye tracking is scaled to 0.0=0 degrees and 1.0=90 degrees - some models could have different xRange values which may affect this time scale.