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>

crates/nu-cmd-base/src/util.rs

@@ -99,7 +99,7 @@ fn get_editor_commandline(
 
 pub fn get_editor(
     engine_state: &EngineState,
-    stack: &mut Stack,
+    stack: &Stack,
     span: Span,
 ) -> Result<(String, Vec<String>), ShellError> {
     let config = engine_state.get_config();