Fix for twigphp/Twig/issues/3685 (leading linebreaks and indentation)

[Sarke]

Reworked the "remove indentation" to properly account for all lines, work with leading linebreaks, and retain codeblocks at start of markdown.

Reworked the tests, to among other things compare with the markdown generated by the markdown library directly.

[stof]

This PR should be send to the Twig repo itself, as this repo is a read-only subtree split. See the extra folder in the Twig repo.

[Sarke]

This PR should be send to the Twig repo itself, as this repo is a read-only subtree split. See the extra folder in the Twig repo.

Thanks, my bad.