[nk-batman21]

With the fenced-code-blocks extra, a fence whose language name contains # or . is not treated as a fence at all. c#, f# and vb.net are Pygments lexer names, and GitHub highlights a ```c# block as C#.

For example:

```c#
#region Setup
var x = 1;
#endregion
```

Some *text* here.

```python
print('hi')
```

On master this renders <p>```c#</p>, then <h1>region Setup</h1> and <h1>endregion</h1>. The closing fence of the C# block then opens a code block that swallows "Some text here." and the ```python line, so code and prose swap places for the rest of the document. This happens with or without Pygments installed.

This adds # and . to the characters allowed in the language name in fenced_code_block_re. A fence with words after the name (```c# title) is still not a fence, and a block with no language whose first line starts with # still gets no language (#396).

Ran: make testone on Python 3.9 and 3.14 (303 tests pass); the new fixture fails without the change.
Checked: ```c# with highlightjs-lang, with safe mode, and inside a list item and a blockquote now renders the same way as ```python.

[nicholasserra]

Thanks!