fix: missing export for public property type

[brianzinn]

I'm unable to build my project with the missing type.
https://forum.babylonjs.com/t/public-property-type-not-exported/45169/5

[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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