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
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documentation](https://github.com/nushell/nushell.github.io) after the
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Co-authored-by: Jelle Besseling <jelle@bigbridge.nl>

crates/nu-command/src/env/config/utils.rs

@@ -71,6 +71,7 @@ pub(crate) fn gen_command(
         arg_keep_raw: vec![false; number_of_args],
         redirect_stdout: false,
         redirect_stderr: false,
+        redirect_combine: false,
         env_vars: env_vars_str,
         trim_end_newline: false,
     }