Remove stack debug assert (#12861)

# Description
In order for `Stack::unwrap_unique` to work as intended, we currently
manually track all references to the parent stack and ensure that they
are cleared before calling `Stack::unwrap_unique` in the REPL. We also
only call `Stack::unwrap_unique` after all code from the current REPL
entry has finished executing. Since `Value`s cannot store `Stack`
references, then this should have worked in theory. However, we forgot
to account for threads. `run-external` (and maybe the plugin writers)
can spawn threads that clone the `Stack`, holding on to references of
the parent stack. These threads are not waited/joined upon, and so may
finish after the eval has already returned. This PR removes the
`Stack::unwrap_unique` function and associated debug assert that was
[causing
panics](https://gist.github.com/cablehead/f3d2608a1629e607c2d75290829354f7)
like @cablehead found.

# After Submitting
Make values cheaper to clone as a more robust solution to the
performance issues with cloning the stack.

---------

Co-authored-by: Wind <WindSoilder@outlook.com>

crates/nu-cli/src/repl.rs

@@ -542,7 +542,9 @@ fn loop_iteration(ctx: LoopContext) -> (bool, Stack, Reedline) {
     let shell_integration_osc633 = config.shell_integration_osc633;
     let shell_integration_reset_application_mode = config.shell_integration_reset_application_mode;
 
-    let mut stack = Stack::unwrap_unique(stack_arc);
+    // TODO: we may clone the stack, this can lead to major performance issues
+    // so we should avoid it or making stack cheaper to clone.
+    let mut stack = Arc::unwrap_or_clone(stack_arc);
 
     perf(
         "line_editor setup",