Feature request: `std::io::Sink` should implement `std::io::IsTerminal` (returning `false`)

[nicola-gigante]

I have a tool which formats output and error messages on the terminal depending on the terminal size and therefore needs to query io::IsTerminal often. I have a trait RenderTarget: io::Write + io::IsTerminal which is used throughout the code.

Today I had the need to pass io::sink() to a function expecting a RenderTarget but io::sink() does not implement io::IsTerminal, and I cannot implement the latter for a wrapper type because it is sealed.

So I'm out of luck. I've found an ugly workaround but I think it would be harmless to have io::sink() implement io::IsTerminal always returning false.