Setup AvaloniaEdit for syntax hightlighting

Markdown.Avalonia and Markdown.Avalonia.SyntaxHigh use AvaloniaEdit for syntax highlighting. Markdown.Avalonia adds AvaloniaEdit styles if an application doesn't have AvaloniaEdit styles.

AvaloniaEdit only provides the style for Avalonia.Themes.Simple for Avalonia.Themes.Fluent in preview6. If your application doesn't use either, Markdown.Avalonia will not be able to add the style. This means that the code with syntax highlighting will not be displayed.

• If App.xaml allows add SimpleTheme or FluentTheme, please add it.

  App.xaml

  <Application xmlns="https://github.com/avaloniaui"
               xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
               xmlns:local="clr-namespace:Markdown.AvaloniaFluentAvaloniaDemo"
               xmlns:sty="using:FluentAvalonia.Styling"
               x:Class="Markdown.AvaloniaFluentAvaloniaDemo.App">
    <Application.Styles>
      <SimpleTheme />
      <sty:FluentAvaloniaTheme />
      <StyleInclude Source="/Assets/AppendingStyles.axaml"/>
    </Application.Styles>
  </Application>

• [experimental: I don't check if this use is legal] add style to MarkdownScrollViewer.Styles

  AnyView.xaml

  <!-- xmlns:md="clr-namespace:Markdown.Avalonia;assembly=Markdown.Avalonia" -->
  <md:MarkdownScrollViewer>
  
    <md:MarkdownScrollViewer.Styles>
      <SimpleTheme />
      <StyleInclude Source="avares://AvaloniaEdit/Themes/Simple/AvaloniaEdit.xaml"/>
    </md:MarkdownScrollViewer.Styles>
  
  </md:MarkdownScrollViewer>