Rename accessor GLTFType to GLTFAccessorType, fix verbose prints, document GLTFAccessor

[aaronfranke]

Follow-up to #36382 and #89317. The name type by itself is too generic and should be avoided.

This PR renames GLTFAccessor get_type to get_accessor_type, renames the GLTFType enum to GLTFAccessorType, moves GLTFAccessorType to gltf_accessor.h, and documents all of GLTFAccessor's properties. It also fixes the verbose prints using a duplicate and incorrect _get_type_name that used lowercase and "float", but the correct values per the GLTF spec are all uppercase and the float one is "SCALAR".

For now the old names are still bound for compatibility, but this change will fix these warnings when building C# assemblies the next time we break compatibility and we remove the old method names:

GltfAccessor.cs(287,18): warning CS0108: 'GltfAccessor.GetType()' hides inherited member 'object.GetType()'. Use the new keyword if hiding was intended.
GltfAccessor.cs(578,43): warning CS0108: 'GltfAccessor.MethodName.GetType' hides inherited member 'object.GetType()'. Use the new keyword if hiding was intended.

[akien-mga]

Thanks!