Support `create function`

[tconbeer]

The most common ddl in our test files is create function. Let's start there.

Postgres: https://www.postgresql.org/docs/15/sql-createfunction.html Snowflake: https://docs.snowflake.com/en/sql-reference/sql/create-function.html BigQuery: https://cloud.google.com/bigquery/docs/reference/standard-sql/user-defined-functions

#262