io: implement is_terminal for stdio types

[maminrayej]

This PR adds the is_terminal function to different stdio types by delegating to the IsTerminal::is_terminal implementation of the standard library. Note that since this trait is sealed, we cannot implement it directly.

Resolves #6407.

[Darksonn]

This will incorrectly return false if stderr is a terminal but there is an ongoing IO operation. Can we instead do this?

Suggested change

	/// Returns true if the descriptor/handle refers to a terminal/tty.
	pub fn is_terminal(&self) -> bool {
	self.std
	.inner()
	.inner()
	.map(|stderr| stderr.is_terminal())
	.unwrap_or_default()
	/// Returns true if the descriptor/handle refers to a terminal/tty.
	pub fn is_terminal(&self) -> bool {
	std::io::stderr().is_terminal()