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/terminal.rs

@@ -0,0 +1,24 @@
+use nu_test_support::{nu, pipeline};
+
+// Inside nu! stdout is piped so it won't be a terminal
+#[test]
+fn is_terminal_stdout_piped() {
+    let actual = nu!(pipeline(
+        r#"
+            is-terminal --stdout
+        "#
+    ));
+
+    assert_eq!(actual.out, "false");
+}
+
+#[test]
+fn is_terminal_two_streams() {
+    let actual = nu!(pipeline(
+        r#"
+            is-terminal --stdin --stderr
+        "#
+    ));
+
+    assert!(actual.err.contains("Only one stream may be checked"));
+}