add option to traceback.format_exception_only to recurse into exception groups

traceback.format_exception_only skips the backtrace and prints only the exception message. For exception groups it would be good to have an option to print the nested exceptions, recursively (just their types and message, no backtrace).

This is motivated by #104150.

Linked PRs

• gh-105292: Add option to make traceback.TracebackException.format_exception_only recurse into exception groups #105294