MD046: Make a fixable rule

[nschonni]

This probably isn't a straight forward one to be fixable. Some of the edges I can think of:

• Sometimes the indented code fences in lists are actually indent errors
• sometimes there are nested backticked fences inside an explicitly indented fence. Would need to decide on a quad or ~ style fence to wrap it
• Language detection would need to be out of scope as it would be very complicated

I'm trying to cleanup https://github.com/mdn/content with a forced

  "MD045": false,
  "MD046": {
    "style": "fenced"
  }

And although it would need manual review on each file anyway, it would make the cleanup much quicker