Fix supportedMethods property type

[1000ch]

According to https://www.w3.org/TR/payment-request/#paymentmethoddata-dictionary, https://www.w3.org/TR/payment-request/#paymentdetailsmodifier-dictionary.

interface PaymentMethodData {
    data?: any;
    supportedMethods?: string[];
}

should be

interface PaymentMethodData {
    data?: any;
    supportedMethods?: string;
}

[mhegazy]

Do not change the .xml files, change https://github.com/Microsoft/TSJS-lib-generator/blob/master/inputfiles/overridingTypes.json instead. also make sure to copy your generated/* into baselines/* and include the new baselines before you submit a change.

[msftclas]

All CLA requirements met.

[1000ch]

@mhegazy Thank you for review.