Fix fenced code blocks in Markdown lexer

[pyrmont]

Many flavours of Markdown permit fenced code blocks. These blocks may begin with three or more ` or ~. The current implementation only permits three. This PR fixes that.

The PR also fixes a bug in code blocks that would match the closing fence even if it occurred within code in the block.

It fixes #1439.

[pyrmont]

@Ravlen If you could have a look over this, that'd be great!

[Ravlen]

@pyrmont I'm not a developer (I'm a technical writer with GitLab, though I noticed I really should have updated my profile -> done) and am not sure how to test this locally, but the regex certainly seems to make sense. Instead of triple backtick/tilde fences, it's 3 or more, and then match the capturing group for the closing fence. Perfectly logical.

Could you double check that this doesn't break when nesting codeblocks (the primary reason for quad fences)? So, a real use case I might have, in our style guide explaining how to use code blocks:

Use a colon to introduce codeblock examples, and use triple backtick fences with
a language tag to make sure the Rouge code coloring will apply:

````markdown
Here is an example of [feature](../feature.md):

```console
$ echo "Sample feature output"
```
````

If this example works too, then I'm 100% thrilled to see the fix! 🙇 🎉 We use Rouge for both GitLab and our docs site, so it will be a big help. I run into edge cases from time to time, and I'll see if I can at least suggest improved Regex next time, if I can figure it out. 👍

[jneen]

+1 from me, looks good on the implementation as well.

[pyrmont]

Thanks all! I've merged it in and this will be part of the v3.17.0 release. That's scheduled to be pushed out on Tuesday 10 March.