Support Inherited Attributes across files

[marklundin]

This PR adds support to extend ESM Scripts in relative modules and inherit their attributes.

This means you can declare a base class in one file, extend it in another and inherit it's attributes.

Fixes #20. Tests included.

// "./rotator.js"
import { Script } from 'playcanvas';

export class Rotator extends Script {
    /**
     * @attribute
     * You can now set the `speed`property dynamically in the editor
     */
    speed = 2;

    update(dt){
        this.entity.rotateLocal(0, this.speed * dt, 0)
    }
}

// "./double-fast-rotator.mjs"
import {Rotator} from './rotator.mjs'

export class DoubleFastRotator extends Rotator {
    update(dt){
        this.entity.rotateLocal(0, this.speed * dt * 2, 0)
    }
}

[mvaligursky]

Awesomeness.