Change FileStream to be async by default

[hez2010]

I believe there's no reason to keep FileStream non-async by default after we rewrote FileStream since .NET 7.
Flipping the switch to true can benefit all APIs like File.Open, File.OpenWrite, File.Create and etc.

[neon-sunset]

Please note that this may degrade single-threaded IO performance on Windows (and I don't think it does anything on Unix?). For some reason, going through Overlapped IO performs much worse.

Further discussion: https://github.com/Cysharp/Utf8StreamReader?tab=readme-ov-file#optimizing-filestream

[stephentoub]

I believe there's no reason to keep FileStream non-async by default

Making this async will cause all synchronous usage to be sync-over-async on Windows. We should not do that.

Thanks for the thought, though.

[hez2010]

Can we instead add overloads which have a FileOptions parameter to those APIs?
It's really convenient to use File.OpenWrite instead of instantiating a FileStream manually.

[stephentoub]

Can we instead add overloads which have a FileOptions parameter to those APIs? It's really convenient to use File.OpenWrite instead of instantiating a FileStream manually.

#24698

[hez2010]

Can we instead add overloads which have a FileOptions parameter to those APIs? It's really convenient to use File.OpenWrite instead of instantiating a FileStream manually.

#24698

That one didn't cover File.OpenWrite and File.OpenRead.