Clarify USV handling in "serialize a JavaScript value to JSON bytes"

[Ms2ger]

"serialize a JavaScript value to JSON bytes" implicitly treats a "string" as a "scalar value string" (in fact, an "I/O queue"), without mentioning how to handle unpaired surrogates.

Fortunately, ECMAScript guarantees that JSON.stringify will return a scalar value string, but that took a moment to verify. It would be nice to see this spelled out in Infra.

[annevk]

I suppose we should update the assert in step 3 of https://infra.spec.whatwg.org/#serialize-a-javascript-value-to-a-json-string? And then maybe add a note below it that it a JSON string might still contain escaped surrogates?