Review #2156 for code improvements: MIN and MAX accept types from a list per aggregate #2696

This is not a bug report. It is an invitation to look at #2156 again, because it adds Date to a list of types each aggregate accepts, where the cause is how values are compared. The goal is to understand the change and decide whether anything should be improved. The notes below come from a quick read of the diff and a few runs, not from a full analysis.

The concerns:

• Each aggregate keeps its own list of accepted types. The generated group code (src/423groupby.js) lets MIN and MAX through for numbers, bigints and now Date; SUM and AVG turn a Date into NULL. Strings are still not on the list: SELECT MAX(d), MAX(s) FROM ? returns the date and leaves the MAX(s) column out entirely.
• One comparison for all aggregates and operators (One comparison module with levels (dates compared by value, MAX/MIN on strings, one NULL order) #2595, planned for 5.0) would give MAX of strings and dates the same answer as ORDER BY, without a type list per aggregate.
• The intermediate value lived in an undeclared global, __alasql_tmp; that part is fixed in Stop ROUND and grouped aggregates from writing a global variable #2694.

Related: #609, #2147, #2155, #2481, #2595, #2694.