Inspector: Added Extension support and revisions

[sunag]

Closes #33196
Related: #33165

Description

The PR makes several revisions in order to add support for extensions, and improves the integration of the TSL Graph, which can now be activated through the UI.

Below is a brief guide on how to add an extension.

Inspector - Settings

extension.json

You need to make a PR in Three.js to add the extension in extension.json. Here are two examples, one using the library’s own code and another externally.

[
	{
		"name": "TSL Graph",
		"url": "./tsl-graph/TSLGraphEditor.js"
	},
	{
		"name": "My Extension",
		"url": "https://raw.githack.com/sunag/three.js/dev-inspector-hello-world/examples/jsm/inspector/addons/MyExtension.js"
	}
]

MyExtension.js

If the objective is to use it externally, always use three/addons so that, through import maps, you use the same classes used at runtime. If the project has new import maps beyond those added in three.js for compatibility reasons, it will be better to use them through the import() function.

A hello world example:

import { Extension } from 'three/addons/inspector/Extension.js';

class MyExtension extends Extension {

	constructor() {

		super( 'My Extension' );

		const div = document.createElement( 'div' );
		div.textContent = 'Hello World';

		this.content.appendChild( div );

		// center the content
		this.content.style.display = 'grid';
		this.content.style.placeItems = 'center';

	}

}

export default MyExtension;

Result