Allow iteration blocks to have an optional extra index parameter (alternative to -n flags) (#6994)

Alters `all`, `any`, `each while`, `each`, `insert`, `par-each`, `reduce`, `update`, `upsert` and `where`,
so that their blocks take an optional parameter containing the index.

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

@@ -119,3 +119,13 @@ fn error_reduce_empty() {
 
     assert!(actual.err.contains("needs input"));
 }
+
+#[test]
+fn uses_optional_index_argument() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"[18 19 20] | reduce -f 0 {|elem accum index| $accum + $index } | to nuon"#
+    ));
+
+    assert_eq!(actual.out, "3");
+}