Support Avalonia Markup files

[ScrubN]

If you want an unsupported language added, provide:

• language name: AXAML (or Avalonia Markup)
• file extension(s): .axaml
• method(s) of commenting text: <!-- --> (same as XAML)
• location of sample code: https://github.com/kikipoulet/SukiUI/blob/b1d624b2472ebd9e442b4870e6e53b8852b86345/SukiUI.Demo/Features/Helpers/HelpersView.axaml

Right now I just use the XAML definition renamed to AXAML:

AXAML
    filter remove_html_comments
    filter call_regexp_common HTML
    extension axaml
    3rd_gen_scale 1.90