Indented fenced code blocks not highlighting in lists

[christhekeele]

Per elixir-makeup/makeup#71:

The correct way (AFAIK) to include a fenced code block (or anything) in a multi-line list item in markdown is by indenting it in between 3 and 7 spaces (I usually go for 4):

- A list

- with bullets

    with multiple lines

- deeply indented

  - nested bullets

    - don't turn into code
    
      - even if you hit
        
        - 8 spaces

          like so

- and fenced code blocks

    ```elixir
    similarly indented
    ````

This (correctly) renders as:

• A list

• with bullets

  with multiple lines

• deeply indented

  • nested bullets

    • don't turn into code

      • even if you hit

        • 8 spaces

          like so

• and fenced code blocks

  similarly indented

However, at least in EXTRAs in my ex_doc projects, I do not get syntax highlighting for mildly indented code blocks in lists.