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-cmd-lang/src/core_commands/do_.rs

@@ -166,7 +166,10 @@ impl Command for Do {
             }
             Ok(PipelineData::ByteStream(mut stream, metadata))
                 if ignore_program_errors
-                    && !matches!(caller_stack.stdout(), OutDest::Pipe | OutDest::Capture) =>
+                    && !matches!(
+                        caller_stack.stdout(),
+                        OutDest::Pipe | OutDest::PipeSeparate | OutDest::Value
+                    ) =>
             {
                 if let ByteStreamSource::Child(child) = stream.source_mut() {
                     child.ignore_error();

crates/nu-cmd-lang/src/core_commands/let_.rs

@@ -63,7 +63,7 @@ impl Command for Let {
 
         let block = engine_state.get_block(block_id);
         let eval_block = get_eval_block(engine_state);
-        let stack = &mut stack.start_capture();
+        let stack = &mut stack.start_collect_value();
         let pipeline_data = eval_block(engine_state, stack, block, input)?;
         let value = pipeline_data.into_value(call.head)?;
 

crates/nu-cmd-lang/src/core_commands/mut_.rs

@@ -63,7 +63,7 @@ impl Command for Mut {
 
         let block = engine_state.get_block(block_id);
         let eval_block = get_eval_block(engine_state);
-        let stack = &mut stack.start_capture();
+        let stack = &mut stack.start_collect_value();
         let pipeline_data = eval_block(engine_state, stack, block, input)?;
         let value = pipeline_data.into_value(call.head)?;
 

crates/nu-cmd-lang/src/example_support.rs

@@ -126,7 +126,7 @@ pub fn eval_block(
     cwd: &std::path::Path,
     engine_state: &EngineState,
 ) -> Value {
-    let mut stack = Stack::new().capture();
+    let mut stack = Stack::new().collect_value();
 
     stack.add_env_var("PWD".to_string(), Value::test_string(cwd.to_string_lossy()));
 
@@ -143,7 +143,7 @@ pub fn check_example_evaluates_to_expected_output(
     cwd: &std::path::Path,
     engine_state: &mut Box<EngineState>,
 ) {
-    let mut stack = Stack::new().capture();
+    let mut stack = Stack::new().collect_value();
 
     // Set up PWD
     stack.add_env_var("PWD".to_string(), Value::test_string(cwd.to_string_lossy()));