Add STD alias to stddevPop function for MySQL compatibility

Why it matters

This feature addresses the need for compatibility with MySQL by providing the commonly used alias STD for calculating the population standard deviation, making it easier for users migrating queries or working in mixed environments.

How to use it

Users can replace stddevPop with the shorter alias STD in their SQL queries to calculate the population standard deviation. For example:

SELECT STD(column_name) FROM table_name;