Description

Closes #6658

With this pr, user can do the following to redirect external commands' stdout/stderr to the same file:

do -i {external command} | save -r out.log --stderr err.log

Or

do -i {external command} | save -r out.log --stderr out.log

And no need to use complete to collect stderr data first, using complete to collect stderr data might results to high memory usage if the external command generates many messages.

Why only works with -r flag

I think we need to consume these two(stdout/stderr) message as soon as possible.

And currently, if we're using save without -r flag, and file have an extension, it'll try to invoke to xxx command, which consumes external commands stdout stream. Finally stderr stream will only be consumed when stdout is handled, which is not good.

The idle way would be using two threads, one for writing stdout, one for writing stderr, then writing these message as soon as possible, like complete, table command.

Why only works with External Stream

Because nu's internal commands just return values, and there is no stdout/stderr property of that value.

Tests

Make sure you've done the following:

• Add tests that cover your changes, either in the command examples, the crate/tests folder, or in the /tests folder.
• Try to think about corner cases and various ways how your changes could break. Cover them with tests.
• If adding tests is not possible, please document in the PR body a minimal example with steps on how to reproduce so one can verify your change works.

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 --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra to check that all the tests pass

Documentation

• If your PR touches a user-facing nushell feature then make sure that there is an entry in the documentation (https://github.com/nushell/nushell.github.io) for the feature, and update it if necessary.