fix: Fallback to 'auto' instead of 'both' for MeshAnnotation

[mrxz]

The specification states that the fallback value for mesh annotations should be auto. In case there is no firstPerson property, it should still be treated as auto. I noticed three-vrm instead used both as fallback. This PR changes the fallback value from both to auto and ensures this is done whether or not there is a firstPerson property.

I could not find out if this applies to VRM0 as well, so I've left _v0Import unchanged.