Add XML highlighting to .axaml (Avalonia XAML)

[colejohnson66]

Used by Avalonia.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?q=extension%3Aaxaml&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/wieslawsoltes/AvaloniaBehaviors/blob/master/samples/BehaviorsTestApplication.Base/Views/MainView.axaml
    • Sample license(s): MIT
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[Alhadis]

• Sample source(s):
  • https://github.com/wieslawsoltes/AvaloniaBehaviors/blob/master/samples/BehaviorsTestApplication.Base/Views/MainView.axaml

This file needs to be added to the samples/XML subdirectory. Samples are used by Linguist's classifier strategy; the more samples we have, the more accurate its analysis.

License checks out: the project whence the sample is taken is MIT-licensed.

[lildude]

Please add a sample as requested by @Alhadis