New Folding for markdown fenced code

Merged Fabian Wunsch requested to merge fabi/syntax-highlighting:markdown-folding into master

While the new style does include a significant amount of duplication, the highlighting in editors like kate feels much more natural. The old problem was that, due to the beginRegion being in a lookahead regex, the size of the beginfold was always 0 characters long, but endfold was 3 or more characters long, leading to a very uneven and irritating highlighting.

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