[Spec] Use USVString rather than DOMString

[stephenmcgruer]

Conversations with i18n representatives revealed that we likely don't need support for unpaired surrogates in the SPC API, and so it is simpler to align on USVStrings (rather than mixing DOMString and USVString).

Fixes https://github.com/w3c/secure-payment-confirmation/issues/208

• [x] Chrome implementation bug: https://crbug.com/1363469
• [x] WPT test changes: N/A (no tests explicitly test unpaired surrogates today, and I don't think there is sufficient value to develop tests that do)

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[stephenmcgruer]

cc @aphillips as FYI

[aphillips]

LGTM