Query CREATE/DROP function supports IF [NOT] EXISTS syntax to match analytics

Added support for the IF NOT EXISTS clause to the CREATE FUNCTION STATEMENT and IF EXISTS to DROP FUNCTION to largely match https://docs.couchbase.com/server/current/analytics/9_udf.htm

i.e.

CREATE FUNCTION name ( args) IF NOT EXISTS body

DROP FUNCTION name IF EXISTS

It is a syntax error to specify OR REPLACE along with IF NOT EXISTS, i.e.

CREATE OR REPLACE name (args) IF NOT EXISTS body 

will result in an error.