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.

src/test_bins.rs

@@ -337,7 +337,7 @@ pub fn nu_repl() {
         let config = engine_state.get_config();
 
         {
-            let stack = &mut stack.start_capture();
+            let stack = &mut stack.start_collect_value();
             match eval_block::<WithoutDebug>(&engine_state, stack, &block, input) {
                 Ok(pipeline_data) => match pipeline_data.collect_string("", config) {
                     Ok(s) => last_output = s,