[Navigation] Update Show Message to use ContentDialog

[nickrandolph]

What would you like to be added:

Change the implementation for ShowMessageDialogAsync to use ContentDialog so that it can be appropriately styled (eg material or fluent)

Why is this needed:

ShowMessageDialogAsync uses the standard message dialog which can't be styled to align with material

For which Platform:

• [ ] iOS
• [ ] Android
• [ ] WebAssembly
• [ ] WebAssembly renders for Xamarin.Forms
• [ ] Windows
• [ ] Build tasks

Anything else we need to know?

[agneszitte]

@nickrandolph what is the status on this one please?

[francoistanguay]

Cant we just deprecate this (or properly document) and recommend for people to use ContentDialog-based prompts?