feat: type writer property as GLTFWriter in VariantMaterialExporterPlugin

[rajkandula]

Description

This PR replaces the any type on the writer property in VariantMaterialExporterPlugin.ts with the concrete GLTFWriter imported from three/examples/jsm/exporters/GLTFExporter.js.

Since @types/three is missing declarations for processMaterialAsync and json on GLTFWriter, this PR utilizes TypeScript module augmentation to safely extend the type. It also adds a type predicate (material is Material) to compatibleMaterial to safely narrow material types within loops and ternary expressions.

This resolves the pending @TODO comment in the exporter plugin file.

Reference Issue

None (resolves a pending inline TODO comment).

Testing

• Verified successful workspace builds using npm run build.
• Ran the test suite via npm run test:ci with all tests passing:
  • @google/model-viewer: 393 passed, 0 failed
  • @google/model-viewer-effects: 13 passed, 0 failed
  • @google/model-viewer-space-opera: 99 passed, 0 failed

[diegoteran]

thanks