Handle Union types

[marklundin]

The parser currently throws an error when parsing attributes with union types.

See also this repro https://playcanvas.com/editor/scene/2086873. Parse the scripts and the editor will throw an error

/** @enum {number} */
const MyEnum = {
    A : 0,
    B : 1,
}

export class Test extends Script {

    /** 
     * @attribute
     * @type {MyEnum|string}
     */
    n = MyEnum.A;
}

What should happen

The parser should at least not throw an error. It could ignore the attribute and warn, or potentially take the first type as the attribute type