Integrate changes from commonmark-java

[kwin]

Commonmark-java has seen quite some releases the past years: https://github.com/commonmark/commonmark-java/releases

On the other hand flexmark-java (a fork of commonmark-java) was not released since more than 2 years and has seen almost no changes. Would it be possible to rejoin forces so that users get the best of both worlds or at least integrate the last fixes from commonmark-java into flexmark-java.

Also a statement on future maintenance of this library would be greatly appreciated.