Support redirect err and out to different streams (#7685)

# Description

Closes: #7364 

# User-Facing Changes

Given the following shell script:
```bash
x=$(printf '=%.0s' {1..100})
echo $x
echo $x 1>&2
```

It supports the following command:
```
bash test.sh out> out.txt err> err.txt
```

Then both `out.txt` and `err.txt` will contain `=`(100 times)

## About the change

The core idea is that when doing lite-parsing, introduce a new variant
`LiteElement::SeparateRedirection` if we meet two Redirection
token(which is generated by `lex` function),
During converting from lite block to block,
`LiteElement::SeparateRedirection` will be converted to
`PipelineElement::SeparateRedirection`.

Then in the block eval process, if we get
`PipelineElement::SeparateRedirection`, we invoke `save` command with
`--stderr` arguments to acthive our behavior.



## What happened internally?
Take the following command as example:
```
^ls out> out.txt err> err.txt
```

lex parsing result(`Tokens`) are not changed, but `LiteBlock` and
`Block` is changed after this pr.

### LiteBlock before
```rust
LiteBlock {
    block: [
        LitePipeline { commands: [
            Command(None, LiteCommand { comments: [], parts: [Span { start: 39041, end: 39044 }] }),
            // actually the span of first Redirection is wrong too..
            Redirection(Span { start: 39058, end: 39062 }, Stdout, LiteCommand { comments: [], parts: [Span { start: 39050, end: 39057 }] }),
            Redirection(Span { start: 39058, end: 39062 }, Stderr, LiteCommand { comments: [], parts: [Span { start: 39063, end: 39070 }] })
        ]
    }]
}
```
### LiteBlock after
```rust
LiteBlock {
    block: [
        LitePipeline { commands: [
            Command(
                None, 
                LiteCommand { comments: [], parts: [Span { start: 38525, end: 38528 }] }),
                // new one! two Redirection merged into one SeparateRedirection.
                SeparateRedirection { 
                    out: (Span { start: 38529, end: 38533 }, LiteCommand { comments: [], parts: [Span { start: 38534, end: 38541 }] }),
                    err: (Span { start: 38542, end: 38546 }, LiteCommand { comments: [], parts: [Span { start: 38547, end: 38554 }] })
                }
        ]
    }]
}
```

### Block before
```rust
Pipeline {
    elements: [
        Expression(None, Expression {
            expr: ExternalCall(Expression { expr: String("ls"), span: Span { start: 39042, end: 39044 }, ty: String, custom_completion: None }, [], false),
            span: Span { start: 39041, end: 39044 },
            ty: Any, custom_completion: None 
        }),
        Redirection(Span { start: 39058, end: 39062 }, Stdout, Expression { expr: String("out.txt"), span: Span { start: 39050, end: 39057 }, ty: String, custom_completion: None }),
        Redirection(Span { start: 39058, end: 39062 }, Stderr, Expression { expr: String("err.txt"), span: Span { start: 39063, end: 39070 }, ty: String, custom_completion: None })] }
```

### Block after
```rust
Pipeline {
    elements: [
        Expression(None, Expression {
            expr: ExternalCall(Expression { expr: String("ls"), span: Span { start: 38526, end: 38528 }, ty: String, custom_completion: None }, [], false),
            span: Span { start: 38525, end: 38528 },
            ty: Any,
            custom_completion: None 
        }),
        // new one! SeparateRedirection
        SeparateRedirection {
            out: (Span { start: 38529, end: 38533 }, Expression { expr: String("out.txt"), span: Span { start: 38534, end: 38541 }, ty: String, custom_completion: None }),
            err: (Span { start: 38542, end: 38546 }, Expression { expr: String("err.txt"), span: Span { start: 38547, end: 38554 }, ty: String, custom_completion: None }) 
        }
    ]
}
```
# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

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

@@ -64,3 +64,69 @@ fn redirect_out() {
         assert!(output.out.contains("hello"));
     })
 }
+
+#[test]
+fn separate_redirection() {
+    use nu_test_support::fs::{file_contents, Stub::FileWithContent};
+    use nu_test_support::playground::Playground;
+    Playground::setup(
+        "external with both stdout and stderr messages, to different file",
+        |dirs, sandbox| {
+            let script_body = r#"
+        echo message
+        echo message 1>&2
+        "#;
+            let expect_body = "message";
+
+            #[cfg(not(windows))]
+            {
+                sandbox.with_files(vec![FileWithContent("test.sh", script_body)]);
+                nu!(
+                    cwd: dirs.test(),
+                    r#"bash test.sh out> out.txt err> err.txt"#
+                );
+            }
+            #[cfg(windows)]
+            {
+                sandbox.with_files(vec![FileWithContent("test.bat", script_body)]);
+                nu!(
+                    cwd: dirs.test(),
+                    r#"cmd /D /c test.bat out> out.txt err> err.txt"#
+                );
+            }
+            // check for stdout redirection file.
+            let expected_out_file = dirs.test().join("out.txt");
+            let actual = file_contents(expected_out_file);
+            assert!(actual.contains(expect_body));
+
+            // check for stderr redirection file.
+            let expected_err_file = dirs.test().join("err.txt");
+            let actual = file_contents(expected_err_file);
+            assert!(actual.contains(expect_body));
+        },
+    )
+}
+
+#[cfg(not(windows))]
+#[test]
+fn redirection_with_pipeline_works() {
+    use nu_test_support::fs::{file_contents, Stub::FileWithContent};
+    use nu_test_support::playground::Playground;
+    Playground::setup(
+        "external with stdout message with pipeline should write data",
+        |dirs, sandbox| {
+            let script_body = r"echo message";
+            let expect_body = "message";
+            sandbox.with_files(vec![FileWithContent("test.sh", script_body)]);
+
+            nu!(
+                cwd: dirs.test(),
+                r#"bash test.sh out> out.txt | describe"#
+            );
+            // check for stdout redirection file.
+            let expected_out_file = dirs.test().join("out.txt");
+            let actual = file_contents(expected_out_file);
+            assert!(actual.contains(expect_body));
+        },
+    )
+}