Add feature for visual.

• Supports customization of language detection for syntax highlighting.
• Auto-scaling column width based on content width.

Supports customization of language detection for syntax highlighting.

It is possible to use a predefined language name as another name language or declare a language name with an xshd resource.

<!-- xmlns:md="clr-namespace:Markdown.Avalonia;assembly=Markdown.Avalonia" -->
<!-- xmlns:mde="clr-namespace:Markdown.Avalonia.SyntaxHigh;assembly=Markdown.Avalonia.SyntaxHigh" -->

<md:MarkdownScrollViewer>
  <md:MarkdownScrollViewer.Plugins>
    <md:MdAvPlugins>
      <mde:SyntaxHiglight>
        <!-- When using a defined language name as another named language name -->
        <mde:Alias Name="ts"  RealName="js" />
        <!-- When declaring new language name with xshd resource -->
        <mde:Alias Name="peg" XSHD="avares://Markdown.AvaloniaDemo/Assets/Pegasus-Mode.xshd" />
      </mde:SyntaxHiglight>
    </md:MdAvPlugins>
  </md:MarkdownScrollViewer.Plugins>
</md:MarkdownScrollViewer>

Auto-scaling column width based on content width.