Light: Unify toJSON() implementations.

[Mugen87]

Related issue: -

Description

Small cleanup to ensure the toJSON() implementations of light classes work in a consistent fashion. All lights behave now like LightProbe and RectAreaLight which means each class is responsible to serialize its own properties. This is a cleaner OO design than checking for properties of subclasses which was previously done in Light.toJSON().

Besides, SpotLight.map is now correctly serialized and copied.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.26 83.06	350.26 83.06	+0 B +0 B
WebGPU	613.73 170.42	613.73 170.42	+0 B +0 B
WebGPU Nodes	612.33 170.15	612.33 170.15	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.25 117.84	482.25 117.84	+0 B +0 B
WebGPU	684.87 186.19	685.12 186.21	+246 B +10 B
WebGPU Nodes	634.71 173.38	634.96 173.39	+246 B +7 B