Box3: Add toJSON, fromJSON

[gkjohnson]

Related issue: #30965 (comment)

Description

A small prototype to add fromJSON and toJSON to Box3 to normalize the serialization process of these shapes.

Note that fromJSON is currently a member function that modifies the already-created Box3 and returns itself, similar to copy. This could instead be a static member function that returns a new Box3 instance.

If this looks good I'll do the same thing for Sphere in a subsequent PR.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.33 78.33	336.33 78.33	+0 B +0 B
WebGPU	547.81 151.89	547.81 151.89	+0 B +0 B
WebGPU Nodes	547.16 151.73	547.16 151.73	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.78 112.28	465.83 112.29	+50 B +10 B
WebGPU	622.7 168.46	622.75 168.46	+50 B +8 B
WebGPU Nodes	577.57 157.72	577.62 157.73	+50 B +3 B