Most editors don't have "first class" Markdown support any more than Geany, they require plugins/packages/bundles, for example:
https://atom.io/packages/search?q=markdown https://github.com/SublimeText-Markdown/MarkdownEditing https://marketplace.visualstudio.com/search?term=markdown&target=VS&... https://wiki.gnome.org/Apps/Gedit/MarkdownSupport https://github.com/textmate/markdown.tmbundle https://plugins.geany.org/markdown.html
The thing they all have in common is being external to the core editor and being _contributed by users_ who needed these features, which is the whole point of having a plugin system in the first place.