Add .clone() method to all query builders

As query builder methods are mutable, there currently is no way to reuse a query multiple times, e.g. this won't work as expected:

```ts
const base = db.select().from(users);
const query1 = base.limit(10);
const query2 = base.limit(20);
```

After adding a `.clone()` method, it would look like this:

```ts
const base = db.select().from(users);
const query1 = base.clone().limit(10);
const query2 = base.clone().limit(20);
```