Support KHR_node_visibility glTF extension

## What problem does this solve or what need does it fill?

I would like to animate visibility of glTF nodes in bevy.

## What solution would you like?

I would like Bevy glTF loader to support the KHR_node_visibility gltf extension.

## What alternative(s) have you considered?

As suggested elsewhere, the workaround for this is applying some transform to the node, usually setting its scale to 0 using constant interpolation.

## Additional context

The KHR_node_visibility extension is soon to be ratified by the Khronos Group, and is tracked on this pull request: https://github.com/KhronosGroup/glTF/pull/2410