Add is-terminal to determine if stdin/out/err are a terminal (#10970)

# Description

I'm not sure if "is-terminal" is the best name for this command as there
is also "term size". Uses
[`is_terminal()`](https://doc.rust-lang.org/stable/std/io/trait.IsTerminal.html#tymethod.is_terminal)
which is cross-platform.

Possible alternative names:
* `term is-tty --stdout`
* `term is-tty stdout`
* `term is-terminal stdout`

If multiple streams are provided an error is returned. The error span
covers all arguments as the incompatible one is not known. This may be
new?

Fixes #10517

# User-Facing Changes

* Add `is-terminal` to check if stdin, stdout, or stderr are a terminal
(TTY)

# Tests + Formatting

The nu tests always redirect stdin, stdout, and stderr so a positive
test case is not possible without extra work

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

The new command will be added automatically

---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

crates/nu-command/tests/commands/mod.rs

@@ -100,6 +100,7 @@ mod split_row;
 mod str_;
 mod table;
 mod take;
+mod terminal;
 mod to_text;
 mod touch;
 mod transpose;