support all kind of executable code blocks in the new styleamrk

[mbehzad]

The original stylemark would allow code blocks for html, js and css. and the content could be in the fenced code block or referenced via a file path (see stylemark spec).
the new stylemark porivdes support for referenced html file, embeded css styles, PR #224 will additionally add support for embedded html content and embeded js.
what would still be missing is referencing css and js files. There can be a valid use case e.g. some lsg components might want to share some css/js for the styleguide presentation (mocking login state, etc) or loading core componets js/css that won't be deployent and therefor no need to bundle/preprocess it. (we might want to gather feedback if developers need this before spending time adding the support)