Add support for UDFs: CREATE FUNCTION

It would be great to have support for `CREATE FUNCTION` statement to define UDFs.

I think it makes sense to develop the more minimal variant of BigQuery first: support for `LANGUAGE` (e.g. javascript) and `AS` (sql) or even sql-only to start with.

https://cloud.google.com/bigquery/docs/reference/standard-sql/user-defined-functions

https://www.postgresql.org/docs/9.1/sql-createfunction.html

先將所要求的最小 CREATE FUNCTION 語法與連結的 BigQuery 和 PostgreSQL 參考進行比較，重點關注 LANGUAGE 和 AS。完成的標準是 parser 接受約定的純 SQL 形式或 LANGUAGE/AS 形式，並一致地拒絕不支援的形式；issue 未指定要執行的檔案或測試。

由索引模型根據 Issue 內容生成。