Expose 'count' property in BatchedMesh

[Kikedao]

Description

BatchedMesh doesn't have a public count property, InstancedMesh does though (https://threejs.org/docs/#api/en/objects/InstancedMesh.count).

Without it it becomes pretty complicated to know how many objects/geometries have been added to a BatchedMesh instance and do operations like traversing all of them to set the matrices, make sure you are in the range before updating one of them, etc.

Solution

I see that the BatchedMesh implementation is already using a private _geometryCount property, it should be as easy as to rename it as count and update the docs.

Alternatives

I think the proposed solution is the best as it would converge the API with InstancedMesh.

Additional context

No response