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

Rust folks have a similar issue, where long long shouldn't be mapped to Int64, which is bridged to BigInt on JS side. https://github.com/rustwasm/wasm-bindgen/issues/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 https://github.com/swiftwasm/WebAPIKit/issues/42#issuecomment-1125355976

先檢視 JSKit 對 Web API IDL long long 值的處理方式，並將 issue 中提到的 AudioData、Blob 和 File 屬性與 wasm-bindgen issue 800 進行比較。要視為完成，需要決定 JavaScript 數字的表示方式，並透過測試確認這些屬性是否支援 BigInt；沒有指定實作檔案或測試。