Fix unexpected flattening of data by par-each (Issue #8497) (#9007)

# Description
Previously, `par-each` acted like a `flatmap`: first mapping the data,
then applying a `flatten`. This is unlike `each`, which just maps the
data. Now `par-each` works like `each` in this regard, leaving nested
data unflattened.

Fixes #8497

# User-Facing Changes
Previously:
`[1 2 3] | par-each {|e| [$e, $e] }` --> `[1,1,2,2,3,3]` 
Now:
`[1 2 3] | par-each {|e| [$e, $e] }` --> `[[1,1],[2,2],[3,3]]`

# Tests
This adds one test that verifies the lack of flattening for `par-each`.

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

@@ -59,6 +59,7 @@ mod network;
 mod nu_check;
 mod open;
 mod p;
+mod par_each;
 mod parse;
 mod path;
 mod platform;

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

@@ -0,0 +1,12 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn par_each_does_not_flatten_nested_structures() {
+    // This is a regression test for issue #8497
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"[1 2 3] | par-each { |it| [$it, $it] } | sort | to json --raw"#
+    ));
+
+    assert_eq!(actual.out, "[[1,1],[2,2],[3,3]]");
+}