feat: expose Caustics materials and types as public API

[nartc]

This PR exposes Caustics materials and types as public API. I didn't document the public API for the change because the current state of the README doesn't state what's exposed as public API. Please let me know if you want this documented in the README.

Why

Caustics export only the Caustics function to create the entire Caustics construct and the update function. This makes it trickier for declarative framework integrations that allow for:

• Updating parameters without recreating Caustics by using their templating language (jsx, html etc...)
• Content Projection / Rendering Children where content/children is supposed to go (as children of the exposed scene)

What

• Expose the materials and their types as public API
• Add createCausticsUpdate function to create an update function that the consumers can call in their animation loop implementations.

Checklist

• [x] Documentation updated
• [x] Ready to be merged

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[github-actions[bot]]

:tada: This PR is included in version 1.19.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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