Add Standard Deviation and Variance as built-in aggregate functions

[Grauenwolf]

• ANSI SQL https://www.oreilly.com/library/view/sql-in-a/9780596155322/ch04s02.html
• SQL Server https://docs.microsoft.com/en-us/sql/t-sql/functions/aggregate-functions-transact-sql?view=sql-server-ver16

Need to research the other databases.