Add isLight: true class property to Light.d.ts

[kohlmannj]

Description

This PR adds the instance property isLight: true to the TypeScript type definitions for the THREE.Light class.

Use Case

I have some code in a project which uses THREE.Scene.traverse() to determine if a given scene (loaded from glTF, in this case) contains light objects. See this Gist for the code: https://gist.github.com/kohlmannj/d64df72eb8d7309ff7120bc9743e95dd

Since the type definition for THREE.Light doesn’t define the isLight property, this TS code has an error:

scene.traverse((object: THREE.Object3D | THREE.Light) => {
  if ('isLight' in object) {
    console.log('Found light in scene', object);
    // This line has a TypeScript error in Three.js r103:
    object.isLight; // ERROR: Property 'isLight' does not exist on type 'never’. ts(2339)
  }
});

That is, I’m trying to use type narrowing to write a type-safe check that a given object is indeed a Light subclass.

—

The implementation of the Light class in Three.js r103 does indeed contain an isLight property, and I have previously used a non-type-safe runtime check like (object as any).isLight to successfully identify objects that are lights.

Therefore, based on the actual implementation, I believe it’s appropriate to add the property isLight: true; to the THREE.Light TypeScript definitions.

Thanks and let me know if you have any questions.

[mrdoob]

Thanks!