bpo-30723: IDLE -- Enhance parenmatch; add style, flash, and help (#2306)

* Add 'parens' style to highlight both opener and closer.
* Make 'default' style, which is not default, a synonym for 'opener'.
* Make time-delay work the same with all styles.
* Add help for config dialog extensions tab, including parenmatch.
* Add new tests. 
Original patch by Charles Wohlganger.