Material#clone: return polymorphic this type

[kungfooman]

Fixes one issue in #3995

Before you had to write:

const a = new StandardMaterial;
const b = a.clone() as StandardMaterial;

Now its just:

const a = new StandardMaterial;
const b = a.clone();

And the type is correctly inferred:

Because the type system can infer the correct type from this: https://www.typescriptlang.org/docs/handbook/advanced-types.html#polymorphic-this-types

JSDoc doesn't support this as type yet, but I made a PR for that too: playcanvas/playcanvas-jsdoc-template#38

I confirm I have read the contributing guidelines and signed the Contributor License Agreement.

[willeastcott]

Great, thanks! 🙏