allow define it as a variable inside closure (#12888)

# Description
Fixes: #12690 

The issue is happened after
https://github.com/nushell/nushell/pull/12056 is merged. It will raise
error if user doesn't supply required parameter when run closure with
do.
And parser adds a `$it` parameter when parsing closure or block
expression.

I believe the previous behavior is because we allow such syntax on
previous version(0.44):
```nushell
let x = { print $it }
```
But it's no longer allowed after 0.60.  So I think they can be removed.

# User-Facing Changes
```nushell
let tmp = {
  let it = 42
  print $it
}

do -c $tmp
```
should be possible again.

# Tests + Formatting
Added 1 test

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

@@ -66,3 +66,10 @@ fn ignore_error_works_with_list_stream() {
     let actual = nu!(r#"do -i { ["a", null, "b"] | ansi strip }"#);
     assert!(actual.err.is_empty());
 }
+
+#[test]
+fn run_closure_with_it_using() {
+    let actual = nu!(r#"let x = {let it = 3; $it}; do $x"#);
+    assert!(actual.err.is_empty());
+    assert_eq!(actual.out, "3");
+}