Publish Avalonia version to NuGet

[ForNeVeR]

To give our users a chance to take a look at the Avalonia version of our library, we need to publish it to NuGet.

I have the following plan:

1. I'll create a separate nuspec for Avalonia in a feature/17-avalonia branch
2. I'll publish the branch as a separate NuGet artifact and mark it as a preview
3. We'll have separate versioning for the master and Avalonia branches for now and will possibly merge the versioning schemes in future
4. That means that Avalonia branch is now set in stone: no rebase for it (because each public release should be strictly reproducible and thus requires a tag)

At that moment, it's important to mark everything as preview or "alpha" so people won't rely on our code too much. Everything inside of that artifact is a subject of very likely changes.

[ForNeVeR]

Decided to name the artifact this way:

<id>AvaloniaMath</id>
<title>Avalonia-Math</title>

[ForNeVeR]

https://www.nuget.org/packages/AvaloniaMath