Fix(duckdb): Generate correct DETACH syntax if IF EXISTS is set

[erindru]

Prior to this PR:

>>> parse_one("DETACH DATABASE IF EXISTS foo", dialect="duckdb").sql(dialect="duckdb")
'DETACH IF EXISTS foo'

This is invalid syntax, at least as of DuckDB 1.3.0. Unlike ATTACH, when IF EXISTS is used for DETACH, it has to be DETACH DATABASE or the following error gets thrown:

Parser Error: syntax error at or near "exists" (Line Number: 1)

This is in line with the DuckDB documentation

Note that DETACH foo works as expected, the error is only thrown if IF EXISTS is added