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-command/tests/commands/math/mod.rs

@@ -509,3 +509,14 @@ fn adding_value_and_list() {
 
     assert_eq!(actual.out, "[1, 3, 5]");
 }
+
+#[test]
+fn adding_tables() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            [[a b]; [1 2]] ++ [[4 5]; [10 11]] | to nuon
+        "#
+    ));
+    assert_eq!(actual.out, "[{a: 1, b: 2}, {4: 10, 5: 11}]");
+}