Allow for stacks to have parents (#11654)

This is another attempt on #11288 

This allows for a `Stack` to have a parent stack (behind an `Arc`). This
is being added to avoid constant stack copying in REPL code.

Concretely the following changes are included here:
- `Stack` can now have a `parent_stack`, pointing to another stack
- variable lookups can fallback to this parent stack (env vars and
everything else is still copied)
- REPL code has been reworked so that we use parenting rather than
cloning. A REPL-code-specific trait helps to ensure that we do not
accidentally trigger a full clone of the main stack
- A property test has been added to make sure that parenting "looks the
same" as cloning for consumers of `Stack` objects

---------

Co-authored-by: Raphael Gaschignard <rtpg@rokkenjima.local>
Co-authored-by: Ian Manske <ian.manske@pm.me>

src/run.rs

@@ -272,7 +272,7 @@ pub(crate) fn run_repl(
     let start_time = std::time::Instant::now();
     let ret_val = evaluate_repl(
         engine_state,
-        &mut stack,
+        stack,
         config_files::NUSHELL_FOLDER,
         parsed_nu_cli_args.execute,
         parsed_nu_cli_args.no_std_lib,