Mesh: Add a setIndexBuffer method

[Popov72]

See https://forum.babylonjs.com/t/how-to-keep-buffers-on-the-gpu-when-using-compute-shaders-for-instancing-or-vertex-data-generation/45951/6

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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