Code blocks don't parse properly when in lists

[MyriaCore]

When the beginning backticks of a codeblock are on the first line of a new list element (numbered, ordered, whatever), markdown-mode fails to parse the codeblock properly, and interprets the ending backticks as starting backticks.

As a result, whenever the user starts a list with a codeblock, then tries to edit it with C-', they'll get an error Not inside a GFM or tilde fenced code block. Also, if the codeblock happens to be a language that has a mode for it in emacs, the fontification fails - the contents of the codeblock are highlighted the default color.

This error is likely due to the parsing rule that requires the starting backticks of a codeblock to have whitespace before it. A correct rule would be whitespace preceding the backticks, or "list bullets" of any type preceding the backticks.

Steps to Reproduce

1. Fill a markdown buffer with the following content:

   1. ```
      this is
      a codeblock
      ```
   2. This is normal text
   

2. Press C-' in the codeblock to edit it
3. See Bug
4. Replace the buffer contents with the following:

   1. ```
      this is
      a codeblock
      ```
   2. This is normal text
   3. ```
      the user thinks this
      is a new codeblock
      I think differently c;
      ```
   

5. Press C-' in the second codeblock
6. See Bug

[syu-id]

I found this bug quite annoying when viewing/editing @logseq's Markdown notes in Emacs. Because all note entries are basically bullets and code fences will break syntax highlighting.

[syohex]

BTW Is this valid markdown syntax ? I couldn't find such examples in commonmark spec. Code block can be indented by spaces but other characters other than spaces can not be put before triple quotes.

[syu-id]

As shown in the issue post, GitHub can render those code fences without a problem. So at least it is supported by GFM (I believe VS Code also supports this).

1. this is
   a codeblock
   

2. This is normal text

3. the user thinks this
   is a new codeblock
   I think differently c;
   

But in the Emacs Markdown mode, they will look like this screenshot: