Fix try not working with let, etc. (#13885)

# Description
Partialy addresses #13868. `try` does not catch non-zero exit code
errors from the last command in a pipeline if the result is assigned to
a variable using `let` (or `mut`).

This was fixed by adding a new `OutDest::Value` case. This is used when
the pipeline is in a "value" position. I.e., it will be collected into a
value. This ended up replacing most of the usages of `OutDest::Capture`.
So, this PR also renames `OutDest::Capture` to `OutDest::PipeSeparate`
to better fit the few remaining use cases for it.

# User-Facing Changes
Bug fix.

# Tests + Formatting
Added two tests.

crates/nu-cli/src/completions/completer.rs

@@ -25,7 +25,7 @@ impl NuCompleter {
     pub fn new(engine_state: Arc<EngineState>, stack: Arc<Stack>) -> Self {
         Self {
             engine_state,
-            stack: Stack::with_parent(stack).reset_out_dest().capture(),
+            stack: Stack::with_parent(stack).reset_out_dest().collect_value(),
         }
     }
 

crates/nu-cli/src/menus/menu_completions.rs

@@ -28,7 +28,7 @@ impl NuMenuCompleter {
         Self {
             block_id,
             span,
-            stack: stack.reset_out_dest().capture(),
+            stack: stack.reset_out_dest().collect_value(),
             engine_state,
             only_buffer_difference,
         }

crates/nu-cli/src/reedline_config.rs

@@ -122,7 +122,7 @@ pub(crate) fn add_menus(
 
             engine_state.merge_delta(delta)?;
 
-            let mut temp_stack = Stack::new().capture();
+            let mut temp_stack = Stack::new().collect_value();
             let input = PipelineData::Empty;
             menu_eval_results.push(eval_block::<WithoutDebug>(
                 &engine_state,