Material: Add missing map properties to toJSON().

[Mugen87]

Fixed #31577.

Description

The PR adds the missing map properties sheenColorMap and sheenRoughnessMap to toJSON().

Since both properties are already supported in MaterialLoader, serialization/deserialization should work now as expected.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.63 79.03	338.63 79.03	+0 B +0 B
WebGPU	567.29 156.89	567.29 156.89	+0 B +0 B
WebGPU Nodes	565.9 156.64	565.9 156.64	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	470.05 113.73	470.27 113.76	+220 B +29 B
WebGPU	638.76 172.87	638.98 172.93	+220 B +61 B
WebGPU Nodes	593.4 162.16	593.63 162.2	+220 B +42 B