Common Table Expressions in PostgreSQL allow a different way to define
derived tables. Here's an example from the pg docs:

	WITH regional_sales AS (
	        SELECT region, SUM(amount) AS total_sales
	        FROM orders
	        GROUP BY region
	     ), top_regions AS (
	        SELECT region
	        FROM regional_sales
	        WHERE total_sales > (SELECT SUM(total_sales)/10 FROM regional_sales)
	     )
	SELECT region,
	       product,
	       SUM(quantity) AS product_units,
	       SUM(amount) AS product_sales
	FROM orders
	WHERE region IN (SELECT region FROM top_regions)
	GROUP BY region, product;

https://www.postgresql.org/docs/current/queries-with.html

This commit adds the :with keyword to the set of keywords allowed to
begin a query against a `prevent_writes` PostgreSQL connection.

Thx to @kamipo, this also adds the same support for sqlite3 and mysql2

https://www.sqlite.org/lang_with.html
https://dev.mysql.com/doc/refman/8.0/en/with.html