TLS: Introduce builtinShadowContext

[sunag]

Related issue: #32378 (comment)

Description

Simplify the use of built-in shadows as used in the webgpu_postprocessing_sss example.

Contexts can be used in cascade, being:

scenePass.contextNode = builtinShadowContext( sssSample, dirLight ).builtinAOContext( aoSample );

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.29 83.05	350.29 83.05	+0 B +0 B
WebGPU	613.1 170.13	613.24 170.19	+138 B +60 B
WebGPU Nodes	611.7 169.85	611.84 169.91	+138 B +58 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.23 117.82	482.23 117.82	+0 B +0 B
WebGPU	684.25 185.94	684.36 186	+109 B +58 B
WebGPU Nodes	634.08 173.15	634.19 173.2	+109 B +53 B