added support for avalonia 12

[KaiIsNA]

This Pull Request updates OxyPlot.Avalonia to support Avalonia 12.0. It addresses breaking changes in the Avalonia API, specifically regarding compiled bindings, internal property changes, and the new diagnostics/developer tools infrastructure.

Breaking Changes

• Projects using this version must target .NET 8 or higher to remain compatible with Avalonia 12.

• Users may need to install the AvaloniaUI.DiagnosticsSupport package to continue using the integrated developer tools.

Testing Performed

• Built and ran the included sample projects on both Windows 11 and Ubuntu.

[jdlowden]

Uploaded to Nuget as Oxyplot.Avalonia12 in the interim. Available at https://www.nuget.org/packages/Oxyplot.Avalonia12. Links to KaiIsNA repo.

@VisualMelon @dephonica This PR has sat here for 2 months. Mind doing a merge? Works perfectly with Avalonia 12 where the current version causes crashes.

[VisualMelon]

@jdlowden sorry for the delay. I did a V12 port a while back so I'll look whether we should merge this one or use mine (I did update all the examples etc.) and hopefully move forward with that fairly soon, but my time and energy are limit at the moment. Please do everyone ping me with annoying "have you shipped it yet?" messages so that I can't forget.

[jdlowden]

@VisualMelon "have you shipped it yet?"

[VisualMelon]

Many of the AvaloniaExamples are broken here; I have a branch that fixes them all master...VisualMelon:oxyplot-avalonia:avalonia-v12

I'm not fussed about ownership, but would prefer we fixed them with this merge, so if you want to incorporate that work into this PR that would be great; if not, or you reckon you're not going to have time in the near future, then we should get this merged (there are some necessary formatting changes but that's not the end of the world) and I'll sort out the examples in other PR.

[VisualMelon]

these appear just to be formatting changes

[VisualMelon]

Actually, I've changed my mind: let's fix them in another PR so that this one is clean (the fixes are messy)

[VisualMelon]

Created #75 on top of this work
Rebase was surprisingly painless
The only change left over in my work is the change to TrackerControl, which we may want to bring into this work if we can understand why I changed it in the first place...