Add packArray() for Matrix types

[ptrgags]

To make the MatrixN API more consistent with CartesianN, I added packArray() and unpackArray(). I also updated the unit tests for matrices to use createPackableSpecs and createPackableArraySpecs() for consistency.

This change is needed for #10116, since EXT_structural_metadata supports arrays of matrices.

@lilleyse could you review?

[cesium-concierge]

Thanks for the pull request @ptrgags!

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[ptrgags]

@lilleyse updated!