feature: Added Mermaid module to support Mermaid charts

[tse-wei-chen]

This package add Mermaid diagram support to Markdown.

How to use

In XAML, add MermaidBlockHandler to MdAvPlugins.

<Window 
	xmlns:mermaid="clr-namespace:Markdown.Avalonia.Mermaid;assembly=Markdown.Avalonia.Mermaid"
>

...another xaml...

<md:MarkdownScrollViewer>
  <md:MarkdownScrollViewer.Plugins>
	<md:MdAvPlugins>
	  <mermaid:MermaidBlockHandler />
	</md:MdAvPlugins>
  </md:MarkdownScrollViewer.Plugins>
</md:MarkdownScrollViewer>

theme support

Mermaid diagrams support light and dark theme. You need to set Theme property in MermaidBlockHandler.

<mermaid:MermaidBlockHandler Theme="dark" />

You can also set BackgroundColor property to make the background transparent.

<mermaid:MermaidBlockHandler BackgroundColor="transparent" />

⚠️ First Run Requirement

The first time a Mermaid diagram is rendered, the library will automatically download a standalone Chromium browser instance (managed by Microsoft.Playwright). This one-time setup requires an active internet connection and may take a few moments depending on your network speed.

Markdown example

```mermaid
graph TD
	A[Start] --> B{Decision}
	B -->|Yes| C[Do X]
	B -->|No| D[Do Y]
	C --> E[End]
	D --> E
``` 

dependencies:

• Markdown.Avalonia.Tight
• Microsoft.Playwright - MIT license
• cdn for mermaid.min.js