Fork our "marked" dependency to the elastic org

[epixa]

marked is an abandoned project that is no longer reliably releasing patch versions of the software. We should fork the project to use in Kibana so we can have control over it for our usage.

This only applies to versions prior to 6.0 since marked should be removed entirely in 6.0.0. See #13425

[uboness]

why not just pulling the codebase into the kibana project... forking it into the elastic org still makes it publicly available right?

[epixa]

@uboness I'm not against this, but we have no precedent for doing this on an entire project, including all the test infrastructure and such. This is something we absolutely want and need to do, but we weren't planning to do it for this particular change.

[uboness]

not sure how much work is required to pull the codebase in... so I trust your judgement. Just wanted to make sure this option is considered. Also, if we do pull it into the elastic org, please put a HUGE warning in the readme that this is purely internal for Kibana and no one else should use it anywhere else.