Fix #5113: Add strikethrough extension to markdown

[bschwede]

Fix #5113

[bschwede]

What do you think about other extensions:

• DescriptionListExtension
• FootnoteExtension
• with update of CommonMark package to v2.8.0: HighlightExtension

[fisharebest]

We currently use ckEditor to edit HTML notes.

We do not have a similar editor for markdown - but I plan to add one.

We should also consider that we may need to change markdown library in the future.
This already happened when the previous library was abandoned.

So, I do not want to deviate too much from the "standard" (i.e. CommonMark).

This will keep our options open for other libraries.

If we add any extensions, then we have to be confident that they will be supported by other libraries.

So, I can't accept this PR until I have time to research which other libraries can also support it.

It would be disappointing if we cannot use an interactive editor because it does not support strikethrough.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.20%. Comparing base (b094da6) to head (4cf6210).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #5274   +/-   ##
=========================================
  Coverage     35.20%   35.20%           
  Complexity    11236    11236           
=========================================
  Files          1159     1159           
  Lines         48103    48104    +1     
=========================================
+ Hits          16934    16935    +1     
  Misses        31169    31169           

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[bschwede]

Thanks for the insides. Your concerns are understandable. It makes sense not to lose sight of the big picture and don't get caught up in feature fever.
On the other hand IMHO some of those additional markdown features would support storytelling very well.
There is, of course, always a risk involved in adapting to other libraries. If a successor library does not support a feature,
the damage in terms of readability in Markdown is not so severe, but it is certainly not ideal. All in all, not easy!

Do you want to use the same editor for html and markdown notes or do you also consider a non-wysiwyg editor just with syntax highlighting?
If the second case is applicable: Do you know tiny-markdown-editor (active since 09/2020)?