[FEATURE] Add syntax for `SELECT` without `FROM`

In Fugue SQL `SELECT` without `FROM` means it will use the previous step output.

However sometimes standard SQL doesn't need `FROM`. for example

```sql
SELECT 1 AS a
```

This is not common but can happen especially in embedded SQL. So we need a way to handle it.

We could add a special syntax: `FROM .` to indicate this statement doesn't have `FROM`.