Fix 1638, adding support dynamic loading marked.sj with fallback for v<6

[johnjbarton]

In v6 jupyter notebook components/marked/lib/marked no longer exists. The extension silently falls over.

Add local copy of marked.min from cdnjs.com into the /python-markdown directory.

require() 'marked' on two paths,
first expecting marked.min.j in the same dir,
second expecting marked in the v6 compenents/ location.

Insert ERROR message in user page if neither is found.

[johnjbarton]

I only verified this works for me on:
jupyter_client : 8.2.0
jupyter_core : 5.3.0
jupyter_server : 2.5.0

[juhasch]

Great, thanks a lot for the PR!
I think it is the right approach to include marked.js directly and not dynamically load it over a CDN.
We should probably include the license for marked from https://github.com/markedjs/marked/blob/master/LICENSE.md.

[johnjbarton]

CDN: the link in the commit description is just doc. The min.js file is in the directory.

license: just copy into the directory? as marked_LICENSE.md?

[johnjbarton]

Now that I understand the system a bit better I removed the dynamic load. The fix is simple now and should work for 5x and 6x.

The marked.min.js file includes these lines:

 * Copyright (c) 2011-2023, Christopher Jeffrey. (MIT Licensed)
 * https://github.com/markedjs/marked

[johnjbarton]

I'm hoping someone can take a look before I forget all the details here.