Add --redirect-combine option to run-external (#8918)

# Description

Add option that combines both output streams to the `run-external`
command.

This allows you to do something like this:

```nushell
let res = do -i { run-external --redirect-combine <command that prints to stdout and stderr> } | complete

if $res.exit_code != 0 {
  # Only print output when command has failed.
  print "The command has failed, these are the logs:"
  print $res.stdout
}
```

# User-Facing Changes

No breaking changes, just an extra option.

# Tests + Formatting

Added a test that checks the new option

# 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.
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Co-authored-by: Jelle Besseling <jelle@bigbridge.nl>

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

@@ -320,3 +320,18 @@ fn quotes_trimmed_when_shelling_out() {
 
     assert_eq!(actual.out, "foo");
 }
+
+#[test]
+fn redirect_combine() {
+    Playground::setup("redirect_combine", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                run-external --redirect-combine sh [-c 'echo Foo; echo >&2 Bar']
+            "#
+        ));
+
+        // Lines are collapsed in the nu! macro
+        assert_eq!(actual.out, "FooBar");
+    });
+}