Handle defaults/themes inside Maui.Core

[PureWeen]

TODO

What should this look like?

• Is it good enough storing to static fields?
• do we need a theme engine?
• Do we try to extract these properties from all the different storage locations at the platform level?

Other libraries are already defining their own default styles. Can we provide a cohesive solution in core?

• https://github.com/dotnet/Comet/tree/6b5748a5462f203b76a38b6daa6cef14f539d1ec/src/Comet/Styles

Should we implement this before/instead of/after this https://github.com/dotnet/maui/issues/7800

[TimLariviere]

It would be nice to have at least a central place with all the defaults declared. They seem tightly tied to the handlers provided by Maui.Core, so having them together in the assembly would make sense.

I'm currently implementing Maui.Core in Fabulous, and I had to look into all BindableProperties of Maui.Controls to extract those defaults.

Right now, I have all the following defaults: https://github.com/fabulous-dev/Fabulous.Maui/tree/main/src/Microsoft.Maui.FabCompat/Handlers/Defaults