Add MermaidExtension for rendering Mermaid.js diagrams

[dereuromark]

Summary

• Adds MermaidExtension that transforms code blocks with language mermaid into Mermaid.js-compatible markup
• Supports <pre> or <div> tag options
• Optional figure wrapper for semantic markup
• Preserves custom attributes and classes
• Comprehensive documentation with JavaScript integration examples

Usage

$converter = new DjotConverter();
$converter->addExtension(new MermaidExtension());

Input:

``` mermaid
graph TD;
    A-->B;
    A-->C;
```

Output:

<pre class="mermaid">graph TD;
    A-->B;
    A-->C;
</pre>

Configuration

Option	Type	Default	Description
tag	string	'pre'	HTML tag ('pre' or 'div')
cssClass	string	'mermaid'	CSS class for Mermaid.js
wrapInFigure	bool	false	Wrap in <figure>
figureClass	string	'mermaid-figure'	Figure wrapper class

Supported Diagram Types

Flowcharts, sequence diagrams, class diagrams, state diagrams, ER diagrams, Gantt charts, pie charts, git graphs, mindmaps, timelines, and more.

See https://mermaid.js.org/ for full Mermaid documentation.

[codecov]

Codecov Report

❌ Patch coverage is 97.56098% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.98%. Comparing base (41bdb80) to head (1187971).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Extension/MermaidExtension.php	97.56%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #131      +/-   ##
============================================
+ Coverage     93.94%   93.98%   +0.03%     
- Complexity     2385     2407      +22     
============================================
  Files            81       83       +2     
  Lines          6310     6379      +69     
============================================
+ Hits           5928     5995      +67     
- Misses          382      384       +2     

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