Markdown: add region folding to syntax folding

[aeschli]

#44795 adds language aware folding to markdown (Thanks a lot!)

Don't forget to also support the folding regions that we already support for markdown:

		"markers": {
			"start": "^\\s*<!--\\s*#?region\\b.*-->",
			"end": "^\\s*<!--\\s*#?endregion\\b.*-->"
		}

Ideally your scanner known where the comment tokens are and can do this more efficiently (and correct) that a regexp on every line.