STL export multiple meshes

[Bulisor]

Add the option to not combine the meshes on export as STL
Based on this discussion
https://forum.babylonjs.com/t/stlexport-could-use-mesh-name-s-instead-than-the-generic-stlmesh-as-solid-label-s/40120

[bjsplat]

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To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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