ShaderNode: Improve syntax (Fluent interface)

[sunag]

Description

This PR adds Fluent interface for ShaderNode
Now is possible to use method chaining between all NodeElements.

I think this can simplify the code reading, besides being common among Three.js users.

/ping @mrdoob

Examples

One line of DFGApprox:

// previous
const r = add( mul( roughness, c0 ), c1 );

// using method chaining
const r = roughness.mul( c0 ).add( c1 );

Simple negate

// previous
reflectVec = negate( positionViewDirection );

// using method chaining
reflectVec = positionViewDirection.negate();

Others examples

//  reflectVec  = transformDirection( reflectVec, cameraViewMatrix );
reflectVec = reflectVec.transformDirection( cameraViewMatrix );

// invert( radianceTextureUVNode.y );
radianceTextureUVNode.y.invert();

This contribution is funded by Google

[sunag]

How it works:

1. Checks that no variable with the element name has been created on the target Node.
2. Returns a closure function, where the function's first argument is the node itself applied to the new element.

// api
myNode.add( 100 );

// shader node (e.g.)
return ( ...params ) => add( myNode, ...params );

[mrdoob]

The new syntax feels much more familiar! 👍👍