fix: preserve recursive CTEs

[nextchamp-saqib]

Issue

In [1]: import ibis

In [2]: query = """WITH RECURSIVE numbers (n) AS (
   ...:     SELECT 1 AS n
   ...:     UNION ALL
   ...:     SELECT n + 1 FROM numbers WHERE n < 5
   ...: )
   ...: SELECT n FROM numbers;"""

In [3]: db = ibis.duckdb.connect()

In [4]: q = db.sql(query)

In [5]: ibis.to_sql(db.sql(query))
Out[5]: 
WITH numbers(n) AS (
  SELECT
    1 AS n
  UNION ALL
  SELECT
    n + 1
  FROM numbers
  WHERE
    n < 5
)
SELECT
  n
FROM numbers

After proposed fix

In [1]: import ibis

In [4]: ...

In [5]: ibis.to_sql(db.sql(query))
Out[5]: 
WITH RECURSIVE numbers(n) AS (
  SELECT
    1 AS n
  UNION ALL
  SELECT
    n + 1
  FROM numbers
  WHERE
    n < 5
)
SELECT
  n
FROM numbers

[cpcloud]

This needs a test.