Add a method to get the raw value of the node

[koros]

This will be used to support the generation of composed type which is made up of primitives:

/**
 * Creates a new instance of the appropriate class based on discriminator value
 * @param parseNode The parse node to use to read the discriminator value and create the object
 * @returns {Primitives}
 */
export function createPrimitivesFromDiscriminatorValue(parseNode: ParseNode | undefined) : number | string | undefined {
    const nodeValue = parseNode?.getNodeValue();
    switch (typeof nodeValue) {
        case "number":
        case "string":
            return nodeValue;
        default:
            return undefined;
    }
}