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-command/src/filesystem/save.rs

@@ -270,7 +270,7 @@ impl Command for Save {
     }
 
     fn pipe_redirection(&self) -> (Option<OutDest>, Option<OutDest>) {
-        (Some(OutDest::Capture), Some(OutDest::Capture))
+        (Some(OutDest::PipeSeparate), Some(OutDest::PipeSeparate))
     }
 }