insert into [...] select [...] returning [...] is not parsed correctly

The following query is valid in SQLite and Postgres:

```sql
insert into test select 1 returning 2;
```

([test](https://www.db-fiddle.com/f/qfSsnPzdrrb227uWmW32mz/0))

sqlparser currently fails to parse this with `ParserError("Expected end of statement, found: 2")`. Apparently, it tries to parse `RETURNING` as a column alias for `1`...