Code block headers containing ``` don't fontify correctly

[darkfeline]

Markdown like this doesn't fontify correctly:

# blah

\``` shell foo bar "```" 
foo

\```

(backslashes for escaping)

To be fair, this syntax isn't formally specified in any markdown spec AFAIK, but gptel uses it for tools calls and I don't think it should be treated as an in-line code markup in any case?

Expected Behavior

Fontified as a code block

Actual Behavior

This gets treated as an in-line code markup:

\``` shell foo bar "```" 

(backslashes for escaping)

Steps to Reproduce

1. Step A
2. Step B
3. Step C

Backtrace

Software Versions

• Markdown Mode: markdown-mode, version 2.8-alpha
• Emacs:
• OS: