allow tables in ++ operator (#7051)

This PR closes #6916, which now allows table/table operations on the
`++` operator.
```
[[a b]; [1 2]] ++ [[a b]; [1 3]]
╭───┬───┬───╮
│ # │ a │ b │
├───┼───┼───┤
│ 0 │ 1 │ 2 │
│ 1 │ 1 │ 3 │
╰───┴───┴───╯
```

crates/nu-parser/src/type_check.rs

@@ -23,7 +23,6 @@ pub fn math_result_type(
     op: &mut Expression,
     rhs: &mut Expression,
 ) -> (Type, Option<ParseError>) {
-    //println!("checking: {:?} {:?} {:?}", lhs, op, rhs);
     match &op.expr {
         Expr::Operator(operator) => match operator {
             Operator::Math(Math::Plus) => match (&lhs.ty, &rhs.ty) {
@@ -84,6 +83,7 @@ pub fn math_result_type(
                         (Type::List(Box::new(Type::Any)), None)
                     }
                 }
+                (Type::Table(a), Type::Table(_)) => (Type::Table(a.clone()), None),
                 _ => {
                     *op = Expression::garbage(op.span);
                     (