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-explore/src/nu_common/command.rs

@@ -96,8 +96,8 @@ fn eval_source2(
     }
 
     let stack = &mut stack.push_redirection(
-        Some(Redirection::Pipe(OutDest::Capture)),
-        Some(Redirection::Pipe(OutDest::Capture)),
+        Some(Redirection::Pipe(OutDest::PipeSeparate)),
+        Some(Redirection::Pipe(OutDest::PipeSeparate)),
     );
     eval_block::<WithoutDebug>(engine_state, stack, &block, input)
 }