Update for Avalonia 11.0.0-preview6

[IhateTrains]

• Bump to Avalonia 11.0.0-preview6
• Use latest preview version Markdown.Avalonia.Tight instead of Markdown.Avalonia package, because we don't need the syntax highlighting functionality: https://www.nuget.org/packages/Markdown.Avalonia.Tight#readme-tab

closes #129

Now it works without throwing an exception:

[ProbablePrime]

Hmm, why do we need to switch the Markdown variant at the same time as this?

Does this resolve the dependency hell?

[IhateTrains]

Markdown.Avalonia.Tight does not depend on AvaloniaEdit, which is not compatible with latest Avalonia preview.

[aldelaro5]

Seems like they just released b1 which is just a superficial version change for version listing whistyun/Markdown.Avalonia@a4849f8

This pr should probably use that one instead.

But yeah, AvaloniaEdit is problematic rn because I tried to upgrade it (I have a WIP pr open), but it is hitting severe perf issues on linux.

[ProbablePrime]

@IhateTrains you bumped the version of "Tight" but as Aldelaro is saying, It is regular Markdown.Avalonia which updated.

[aldelaro5]

@IhateTrains you bumped the version of "Tight" but as Aldelaro is saying, It is regular Markdown.Avalonia which updated.

Sorry for the confusion, but actually, ONLY Tight has its nuget updated to b1 right now. We still don't want regular because of AvaloniaEdit.

[ProbablePrime]

Ahh no worries, Very confusing matter here. I'll leave you guys to it.