citeproc-java offers Markdown output -> michel-kraemer/citeproc-java#252

This could be more advanced than a simple HTML to markdown conversion we implemented in #13387.

Task:

• Add unit tests to show the citeproc-java markdown conversion results (below processMarkdownAsMarkdown in jabgui/src/test/java/org/jabref/gui/preview/ClipboardContentGeneratorTest.java. This is more a documentation for JabRef developers than a real test.
• at processMarkdown, introduce a check if CSL is used. If yes, use CiteProcs conversion. If not, use Flexmark.
  • Hint: Add markdown as output format michel-kraemer/citeproc-java#252 (comment)

Could be a bit more complicated due to visiblity, etc.

Note: This is a follow-up to #12552