Handle cancellation of TextReader/TextWriter access resiliently

[alexrp]

The TextReader/TextWriter and Stream properties exposed on the TerminalReader/TerminalWriter and TerminalHandle classes, respectively, are meant to be long-lived and resilient. For the Stream property, this works as expected; disposal does nothing and cancellation has no lasting effect (other than potentially losing some input/output that was scheduled).

Unfortunately, System.IO.StreamReader/System.IO.StreamWriter are not guaranteed to be in a valid state after a cancellation has occurred. This means that our properties can be rendered unusable if the user ever performs cancellation.

The main reason we have these properties is line-based input:

cathode/src/core/IO/TerminalReader.cs

Lines 69 to 77 in 695b297

	public string? ReadLine()
	{
	return TextReader.ReadLine();
	}
	public ValueTask<string?> ReadLineAsync(CancellationToken cancellationToken = default)
	{
	return TextReader.ReadLineAsync(cancellationToken);
	}

(Other than the above, these properties are not used in Cathode itself.)

The only solution I see here is to implement our own derived System.IO.TextReader/System.IO.TextWriter classes specialized for our needs. We can then do whatever we need to do to keep the state valid across cancellations.