Add MONTHNAME functon to PyDough

[knassre-bodo]

Goal: Add a MONTHNAME function to PyDough which takes in a Date/Timestamp value and returns the 3-letter abbreviation of the month name as a string (e.g. "Jan", "Feb", "Mar", etc.)

Specific dialects have ways to do this, but the general base structure can be done by extracting the MONTH and doing a case statement like so: CASE MONTH(d) WHEN 1 THEN 'Jan' WHEN 2 THEN 'FEB' ...

This feature need to be documented in the function list documentation