Visualize VoxelCollisionMap in Meshcat

[siddancha]

This PR adds a native Meshcat visualization of planning::VoxelCollisionMap using THREE.js' InstancedMesh of BoxGeometries.

• Implements internal::InstancedMeshData to msgpack and set InstancedMesh objects in Meshcat.
• Implements SetObject(BoxList) to set a list of boxes as an InstancedMesh object.
• Implements SetObject(VoxelCollisionMap) as two BoxLists -- one for occupied cells (defaults to red) and another or unknown cells (defaults to semi-transparent black).

In addition, I had to do the following as temporary measures:

• Expose planning/dev publicly so that it could be added as a dependency to Meshcat.

Test

This PR also adds a meshcat_voxel_grid_manual_test (along similar lines as meshcat_manual_test) to manually visualize a VoxelCollisionMap in Meshcat.

Run:

bazel test //geometry:meshcat_voxel_grid_manual_test

• A 10x10x10 grid starts out with all unknown voxels:

  

• Then, the bottom rows are updated to be occupied (red) and the middle rows are updated to be free space (no boxes are drawn for free voxels).

  

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This change is 

[jwnimmer-tri]

+(status: do not review) while we discuss in Slack.

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