Examples: Move jsm/nodes to ES6.

[Mugen87]

Related issue: -

Description

Since devs still work at this part of the repository, it's actually better to move it to ES6, too.

[mrdoob]

Which should be merged first? This one or #21800?

[donmccurdy]

Let's merge this PR first, I can clean up the other one afterward. Will be glad to have ES6 Class syntax here! 👍

[Mugen87]

@sunag I have not converted the following line to ES6:

three.js/examples/jsm/nodes/core/TempNode.js

Lines 68 to 70 in b13eccc

	name = TempNode.prototype.generate.call( this, builder, output, uuid, data.output, ns );
	const code = this.generate( builder, type, uuid );

I'm not sure why generate() is called twice in different ways? 🤔

[sunag]

A long time ago I made this block of code, it is need of some revisions...
The use prototype is to prevent call the extended abstract function without call the super.generate().

I don`t test it but we could do something different now, like this for example:

..

name = this.generateTempVar( builder, output, uuid, data.output, ns );

const code = this.generate( builder, type, uuid );

..

generateTempVar( builder, output, uuid, type, ns ) {

	if ( ! this.getShared( builder, output ) ) console.error( 'THREE.TempNode is not shared!' );

	uuid = uuid || this.uuid;

	return builder.getTempVar( uuid, type || this.getType( builder ), ns, this.getLabel() ).name;

}

generate( builder, output, uuid, type, ns ) {

	return this.generateTempVar( builder, output, uuid, type, ns );

}

[mrdoob]

Thanks!