Nodes: Rework ShaderNodeElements and make some other refactorings

[LeviPesin]

Fixes #25240, partially fixes #23666
Implements #25240 (comment) and #25240 (comment)

Description

Changes:

• Remove ShaderNodeElements and ShaderNodeBaseElements -- now the corresponding functions are exported by nodes files themself. Nothing changes for importing from three/nodes,
• Now some nodes assume that all nodes passed to them are in nodeObject form (user should manage that, but it's mostly managed by the nodes system),
• Introduce addNodeElement/addLightNode/addNodeClass/addNodeMaterial functions,
• Some refactoring introducing more of node chaining (it especially greatly simplifies functions) in most nodes files (examples can be refactored in a separate PR),
• Rename VarNode.add/sub/mul/div() to ShaderNode.addAssign()/subAssign()/etc (with semantics changed very slightly).

Tested all examples -- they seem to work.

[LeviPesin]

/ping @sunag @epreston This PR should resolve #25240.

[sunag]

@LeviPesin Awesome! It just broke webgpu_lights_custom example. I'm investigating...

[sunag]

Seems that we have just one last circular dependency in NodeLoader.js

npx madge examples/jsm/nodes/Nodes.js --circular --warning

[LeviPesin]

I think we can move nodeLib and fromType to a separate file and this should resolve the issue... Will do that.

[LeviPesin]

Done!

[sunag]

@LeviPesin We should give the option to expand the serialization, Node.fromType() is a good API, I think we should use a nodeLib = new WeakMap inside Node.js add method like addNodeClass( SomeNode ) and addNodeElement().

[LeviPesin]

Agreed! And same for materials.
Will do that soon...

[LeviPesin]

Done!
Remaining questions:

• What are the best names for addNodeElement/addLightNode/addNodeClass/addNodeMaterial? Maybe register, not add?
• Shouldn't we move createNodeFromType and createMaterialFromType to corresponding classes so that they would be static methods?
• Shouldn't we also move addNodeElement/addLightNode/addNodeClass/addNodeMaterial to classes?

[sunag]

@mrdoob do you have some suggestion about this #25498 (comment)? I'm thinking of merge and we deciding these definitions soon.

[sunag]

Modifications are more internal, so not affect the external API (examples/projects) or very little. Merging...