Add a new split_comma_separated() utility function.
Review Request #11575 — Created April 5, 2021 and submitted
This function standardizes splitting a comma-separated string into a
list of values, taking care to remove all extra whitespace and handling
multiple commas in a row.
The base file pattern support has been updated to use this, and some
upcoming tool work will benefit from it as well.
Unit tests pass on Python 2.7 and 3.x.
Used this function in the upcoming shellcheck work.