Feature summary (what you would like to be able to do and where):
The mw.text.listToText() feature should be able to produce lists with serial commas, e.g. "1, 2, and 3"

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Currently, the mw.text.listToText() feature produces lists such as "1 and 2" or "1, 2 and 3", with no option for a serial comma before the conjunction. Most American style guides mandate the use of a serial comma before the and for lists of three or more items.

Benefits (why should this be implemented?):
Currently, making lists with a serial comma in lua currently requires either re-implementing a custom version of the listToText() function or workarounds such as specifying different conjunctions based on the length of the list. Having this option in the function would greatly simplify the code of modules that produce lists.