Add support for scripting statements
```sql
CASE product_id
  WHEN 1 THEN SELECT 1;
  WHEN 2 THEN SELECT 2;
  ELSE SELECT 3;
END CASE;
```

```sql
IF EXISTS(SELECT 1) THEN SELECT 1;
  ELSEIF EXISTS(SELECT 2) THEN SELECT 2;
  ELSE SELECT 3;
END IF;
```

[BigQuery CASE](https://cloud.google.com/bigquery/docs/reference/standard-sql/procedural-language#case)

[BigQuery IF](https://cloud.google.com/bigquery/docs/reference/standard-sql/procedural-language#if)

[Snowflake CASE](https://docs.snowflake.com/en/sql-reference/snowflake-scripting/case)
[Snowflake IF](https://docs.snowflake.com/en/sql-reference/snowflake-scripting/if)