nodeBuilder: Add abstract reference for buildFunctionCode

[s-rigaud]

Description

this.buildFunctionCode is referenced inside NodeBuilder.buildFunctionNode() and defined in both subclasses so an abstract reference could be nice.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.02 78.26	336.02 78.26	+0 B +0 B
WebGPU	523.99 145.48	524.05 145.49	+55 B +7 B
WebGPU Nodes	523.46 145.38	523.51 145.38	+55 B +3 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.06 112.14	465.06 112.14	+0 B +0 B
WebGPU	594.38 161.08	594.43 161.08	+55 B +3 B
WebGPU Nodes	549.5 150.51	549.55 150.52	+55 B +3 B