RichTextBlock bindings

[sleepyfran]

There's a new kid in town in Avalonia v11 that allows for inline text blocks formatting, as introduced in these couple of PRs: https://github.com/AvaloniaUI/Avalonia/pull/6953 https://github.com/AvaloniaUI/Avalonia/pull/8370

Currently working in getting some initial bindings working, since it doesn't follow the usual control with simple properties but it introduces the new TextElement that looks like a control but isn't, it's a bit more cumbersome to create than an usual binding.

Just opening this to create awareness + if someone else is working on the same we can team up to get this done 😄