Make exit code available in catch block (#12648)

# Description
Bandaid fix for #12643, where it is not possible to get the exit code of
a failed external command while also having the external command inherit
nushell's stdout and stderr. This changes `try` so that the exit code of
external command is available in the `catch` block via the usual
`$env.LAST_EXIT_CODE`.

# Tests + Formatting
Added one test.

# After Submitting
Rework I/O redirection and possibly exit codes.

crates/nu-command/tests/commands/try_.rs

@@ -93,3 +93,9 @@ fn can_catch_infinite_recursion() {
         "#);
     assert_eq!(actual.out, "Caught infinite recursion");
 }
+
+#[test]
+fn exit_code_available_in_catch() {
+    let actual = nu!("try { nu -c 'exit 42' } catch { $env.LAST_EXIT_CODE }");
+    assert_eq!(actual.out, "42");
+}