Add tee command for operating on copies of streams (#11928)

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# Description
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This is inspired by the Unix tee command, but significantly more
powerful. Rather than just writing to a file, you can do any kind of
stream operation that Nushell supports within the closure.

The equivalent of Unix `tee -a file.txt` would be, for example, `command
| tee { save -a file.txt }` - but of course this is Nushell, and you can
do the same with structured data to JSON objects, or even just run any
other command on the system with it.

A `--stderr` flag is provided for operating on the stderr stream from
external programs. This may produce unexpected results if the stderr
stream is not then also printed by something else - nushell currently
doesn't. See #11929 for the fix for that.

# User-Facing Changes
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If someone was using the system `tee` command, they might be surprised
to find that it's different.

# Tests + Formatting
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# After Submitting
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crates/nu-command/tests/commands/mod.rs

@@ -104,6 +104,7 @@ mod split_row;
 mod str_;
 mod table;
 mod take;
+mod tee;
 mod terminal;
 mod to_text;
 mod touch;

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

@@ -0,0 +1,49 @@
+use nu_test_support::{fs::file_contents, nu, playground::Playground};
+
+#[test]
+fn tee_save_values_to_file() {
+    Playground::setup("tee_save_values_to_file_test", |dirs, _sandbox| {
+        let output = nu!(
+            cwd: dirs.test(),
+            r#"1..5 | tee { save copy.txt } | to text"#
+        );
+        assert_eq!("12345", output.out);
+        assert_eq!(
+            "1\n2\n3\n4\n5\n",
+            file_contents(dirs.test().join("copy.txt"))
+        );
+    })
+}
+
+#[test]
+fn tee_save_stdout_to_file() {
+    Playground::setup("tee_save_stdout_to_file_test", |dirs, _sandbox| {
+        let output = nu!(
+            cwd: dirs.test(),
+            r#"
+                $env.FOO = "teststring"
+                nu --testbin echo_env FOO | tee { save copy.txt }
+            "#
+        );
+        assert_eq!("teststring", output.out);
+        assert_eq!("teststring\n", file_contents(dirs.test().join("copy.txt")));
+    })
+}
+
+#[test]
+fn tee_save_stderr_to_file() {
+    Playground::setup("tee_save_stderr_to_file_test", |dirs, _sandbox| {
+        let output = nu!(
+            cwd: dirs.test(),
+            "\
+                $env.FOO = \"teststring\"; \
+                do { nu --testbin echo_env_stderr FOO } | \
+                    tee --stderr { save copy.txt } | \
+                    complete | \
+                    get stderr
+            "
+        );
+        assert_eq!("teststring", output.out);
+        assert_eq!("teststring\n", file_contents(dirs.test().join("copy.txt")));
+    })
+}