Add support for writing to stderr

[glen-84]

I'm wanting to use Spectre.Console along with System.CommandLine, so I'm creating an adapter, but implementing IStandardStreamWriter Error is not really possible since Spectre doesn't directly support stderr without creating another instance of AnsiConsole.

Also, as mentioned here, stderr should really be used when outputting errors.

Would it be possible to add support for stderr within a single AnsiConsole instance, so that you could use, for example, ansiConsole.Error.Write(table), etc.?