Markdown filter indexes UTF-8 strings with countUntil result

[MartinNowak]

For example filterMarkdown("[ö](/o)") is not recognized as link.

The problem is, that countUntil returns the number of code points.
https://github.com/rejectedsoftware/vibe.d/blob/master/source/vibe/textfilter/markdown.d#L796

[s-ludwig]

Thanks for reminding me that this needed to be fixed. At the time when std.algorithm.indexOf was deprecated, I just blindly followed the suggestion to use countUntil and didn't realize the semantic issues (and I think countUntil was acually returning a byte index until recently...)