feat: add ApplePayButtonType.continue_ support on iOS 15.0+

[stephenmcd]

Expose PKPaymentButtonType.continue through the ApplePayButtonType enum as ApplePayButtonType.continue_ to avoid Dart keyword clash. Map the new value to PKPaymentButtonType.continue behind an #available(iOS 15.0, *) guard, falling back to .plain on older OS versions.

Closes #328

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