WebXRManager: Refactoring and added connected/disconnected controller events.

DOF Example: Use Scene.background.

GLTFExporter: Avoid error on materials without emissive

TransformControls: Add domElement property.

To match the documentation: https://threejs.org/docs/#examples/en/controls/TransformControls
TransformControls should expose a `domElement` property.

````
.domElement : HTMLDOMElement
The HTMLDOMElement used to listen for mouse / touch events. This must be passed in the constructor; changing it here will not set up new event listeners.
````

TS: Added map definition to MeshDepthMaterial and MeshDistanceMaterial.

Editor: Fix Geometry sidebar.

Editor: Fix ui.three.js.

Examples: Clean up.

Add support for linting example scripts

Docs: Added Scene.environment description.

CubeTexturePass: Fix usage of cube shader.

SubdivisionModifier: Fix runtime error with empty uvs.