[w3c/webpayments] Sepamail WebIDL syntax and algorithms (#219)

I suggest the following changes in in [sepamail proposal](https://w3c.github.io/webpayments/proposals/sepamail):

1. The type of `supportedNetworks` should change from `sequence` to `sequence<DOMString>`.
1. The type of `supportedTypes` should change from `sequence` to `sequence<BasicSepamailtype>`.
1. The name of `SelectedNetwork` should change to `selectedNetwork`.
1. The name of `SelectedType` should change to `selectedType`.

Also, please add some algorithms to process `BasicSepamailRequest` in the browser. At the very least, please specify what should happen if `supportedNetworks` or `supportedTypes` is an empty list. What should happen if `supportedNetworks` contains empty or duplicate strings. (I suggest throwing TypeError in all those cases.)

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https://github.com/w3c/webpayments/issues/219