Test: Markdown extension

[kieferrm]

Test plan for #6106

• Windows - @bpasero
• OS X - @sandy081
• Linux - @aeschli

Verify

• syntax highlighting in markdown source in all built-in themes
• working markdown preview in all built-in themes
• "open preview" file explorer context menu contribution
• "Show preview", "Open Preview to the Side", "Show source" editor action bar contributions
• keybindings for open/toggle preview
• test custom styles (see markdown.styles missing should not break markdown preview #6688)
• test custom styles paths including '#' (see Markdown styles: Can't use style sheet that contains "#" in the local path #3613)

[bpasero]

@kieferrm there are some issues with the coloring in markdown sources. I stopped filing issues because I am also not sure if our repository is the right place for them.