[Markdown] Reduce unique fenced codeblock scopes (sublimehq#4229)

This commit...

1. removes syntax name from `meta.code-fence.definition.[begin|end].[syntax]`.

   As a result each code block uses common scope:
      `meta.code-fence.definition.[begin|end].markdown-gfm`

   Amount of unique scopes which need to be handled by ST is reduced.

2. adds `meta.code-fence.body` to content part, so code fences can be addressed
   or searched by `meta.code-fence` scope.

   Example:

       view.sel().add_all(view.find_by_selector("meta.code-fence"))

   selects all fenced code blocks in a document.

Author: deathaxe