find a cleaner way to extend ParsedownExtra with multiple extensions

[raffaelj]

I use ParsedownCheckbox and ParsedownToC extension. To let them work together, I had to use a dirty and unpredictable trick:

Because of the weird extension behaviour of Parsedown, I had to copy/paste the original ParsedownCheckbox class with a namespace modification. So running composer update will not effect that class without manual adjustments.

use \ParsedownToC as ParsedownExtra;
class ParsedownCheckbox extends ParsedownExtra

see: https://github.com/raffaelj/cockpit_BetterMarkdown/blob/master/Helper/Markdown.php#L158-L174

Parsedown 2.0 will have a cleaner method, but it is not released, yet.

The current solution could also break, if these extensions share similar methods and variables in a future update.

[raffaelj]

fixed with c83d0a2