NodeBuilder: Introduce addFlowCodeHierarchy() / NodeBlock

[sunag]

Related issue: #29489

Description

Improves support for code block and fixes issues related to conditionals.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	687.46 170.22	687.46 170.22	+0 B +0 B
WebGPU	839.72 225.2	840.46 225.41	+745 B +212 B
WebGPU Nodes	839.23 225.08	839.97 225.29	+745 B +210 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	463.24 111.86	463.24 111.86	+0 B +0 B
WebGPU	533.78 143.98	534.61 144.23	+835 B +251 B
WebGPU Nodes	490.12 133.72	490.96 133.98	+835 B +255 B

[mrdoob]

Do we still need temp()?

[sunag]

Do we still need temp()?

Certainly not!