Switch to `FileStream`/`RandomAccess` for I/O if feasible (.NET 9+)

[alexrp]

A number of improvements were made to FileStream in .NET 6: https://devblogs.microsoft.com/dotnet/file-io-improvements-in-dotnet-6

FileStream is now looking like a more attractive option for I/O compared to the hand-rolled interop we do right now.

That said, there are three major issues:

• Windows: For as long as we need the UTF-8 input workaround (see Switch to UTF-8 input on Windows #68), we will need to keep a code path that doesn't use FileStream since we need to use ReadConsole as opposed to ReadFile in that case. No longer an issue as this workaround was removed.
• Unix: There is no first-class support for asynchronous operations and cancellation in FileStream yet. This would make it a functional downgrade over what we have now with UnixCancellationPipe.
  • Implement io_uring support for FileStream dotnet/runtime#51985
  • An idea for supporting cancellation in RandomAccess on Unix dotnet/runtime#96530).
• RandomAccess: It would be ideal to use RandomAccess directly, but this is not currently possible due to arbitrary type requirements.
  • Relax RandomAccess type requirements, make all Read*|Write* methods work with non-seekable files dotnet/runtime#58381

The (eventual) benefits of the switch would be improved performance, 'free' support for OS-native asynchronous operations, cancellation support, and more maintainable terminal driver code.