Adds Support for Enum refs

[marklundin]

Adds support for enum attributes by reference. Fixes #12 .

Both the following using literal and reference are now supported. Tests updated to include both use cases.

import { Script } from 'playcanvas'

/** @enum {number} */
const Lights = {
    ON: 1,
    OFF: 0,
    UNKNOWN: 0.5
}

class Hack extends Script {
    /**
     * @attribute
     * @type {Lights}
     */
    ambient = Lights.OFF
    
    /**
     * @attribute
     * @type {Lights}
     */
    spec = 0
}

export { Hack };