Browser version support in platform checks

[MaxDesiatov]

When writing requirements for swiftwasm/JavaScriptKit#56, it reminded me of my old idea for browser version checks in Swift:

@available(Safari 14, Edge 79, Firefox 68, Chrome 67)
final class JSBigInt {
  //...
}

Then user code could look like:

if #available(Safari 14, Edge 79, Firefox 68, Chrome 67) {
  // code that uses `JSBigInt`
else {
  // backward-compatible code
}

Ideally, one would write:

if #available(JSBigInt) {
  // code that uses `JSBigInt`
else {
  // backward-compatible code
}

but this doesn't match the existing semantics of #available, which checks for platforms, not features. And making the toolchain aware of every feature is not realistic in my opinion. Besides, AFAIR Xcode (and hopefully SourceKit-LSP) fix-its propose adding corresponding platform version checks automatically after you attempt to use a "future" API when targeting older platforms.