`JSNumber` type to support `long long` type in Web API IDL

[MaxDesiatov]

Rust folks have a similar issue, where long long shouldn't be mapped to Int64, which is bridged to BigInt on JS side. rustwasm/wasm-bindgen#800

Checked a few other places where long long is used, like AudioData, Blob, and File and all of them return a JS number in corresponding properties, not BigInt. I'm 95% sure none of these support BigInt, remaining 5% certainty could be added by writing actual tests. I personally vote for either keeping these Int32 as proposed, or creating a new union type. Just riffing:

enum JSNumber: ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral {
case integer(Int32)
case float(Double)
// ...
}

Should we add such type to JSKit then?

cc @kateinoigakukun

Originally posted by @MaxDesiatov in swiftwasm/WebAPIKit#42 (comment)