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-plugin-test-support/src/plugin_test.rs

@@ -156,7 +156,7 @@ impl PluginTest {
         };
 
         // Eval the block with the input
-        let mut stack = Stack::new().capture();
+        let mut stack = Stack::new().collect_value();
         let data = eval_block::<WithoutDebug>(&self.engine_state, &mut stack, &block, input)?;
         if matches!(data, PipelineData::ByteStream(..)) {
             Ok(data)