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/all.rs

@@ -108,6 +108,16 @@ fn early_exits_with_0_param_blocks() {
     assert_eq!(actual.out, "1false");
 }
 
+#[test]
+fn uses_optional_index_argument() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"[7 8 9] | all {|el ind| print $ind | true }"#
+    ));
+
+    assert_eq!(actual.out, "012true");
+}
+
 #[test]
 fn unique_env_each_iteration() {
     let actual = nu!(