Add export for BuiltinNode in TSL.js

[Methuselah96]

Related issue: N/A

Description

Fixes #31615 (comment). TSL.builtin wasn't being exported from Three.TSL.js, and therefore wasn't defined on TSL.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.1	338.91 79.1	+0 B +0 B
WebGPU	578.07 159.22	578.08 159.23	+11 B +6 B
WebGPU Nodes	576.68 158.98	576.69 158.99	+11 B +6 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.7 113.91	470.7 113.91	+0 B +0 B
WebGPU	648.61 175.26	648.61 175.26	+0 B +0 B
WebGPU Nodes	602.71 164.39	602.71 164.39	+0 B +0 B

[Mugen87]

Exporting builtin() is required e.g. when third-party clipping code what's to access clip distances over the builder. Just want to name at least one use case^^.