Improve markdown implementation for italic/bold handling (edge cases)

[marijnh]

(Originally reported by @Mobiletainment as ProseMirror/prosemirror#730)

Issue details

When putting italic and bold text directly next to each other, there's a good chance that it gets interpreted wrongly due to weaknesses in markdown's italic and bold formatting.

Steps to reproduce

Take the following markdown for example:

*italic text directly next to ***bold text**

• Expected output: italic text directly next to bold text
• Actual output: *italic text directly next to *bold text

Please specify which version of ProseMirror you're running

ProseMirror v0.10.1
ProseMirror example at http://prosemirror.net/examples/markdown/ (as of 28th November 2017)

Affected platforms

• all

Screenshots / Screencast (Optional)

I tested this behavior with other markdown implementations and saw that this edge case is properly handled by some, e.g. pandoc: http://johnmacfarlane.net/babelmark2/?text=*italic+text+directly+next+to+***bold+text**

[marijnh]

The problem is that a ProseMirror document can represent documents that can't be represented in Markdown text. CommonMark is pretty clear on where a delimiter must be parsed as closing strong or emphasized text, and directly after whitespace it shouldn't.

We could try to add magic to the Markdown serializer to fix up sitations like this by moving the space outside of the marked range. I don't think that can be done perfectly, but even losing some information is better than outputting bogus Markdown text.