Show platform UI for alert/prompt in webviews

### Description

This probably should not happen and apps should not do this, but we added a feature in Xamarin.Forms that appears to have value to some people: https://docs.microsoft.com/en-us/xamarin/xamarin-forms/platform/windows/webview-javascript-alert

This current feature only talks about Windows, but I see the Blazor team has done things for iOS. For this reason, we need to clarify what we want to allow and how we want to handle it. If apps are not supposed to use alert/prompt, then why is blazor using specific code - and why not all the platforms? If we want to use this, then we need to make sure it works in all webviews.

Related: windows prompts look bad: https://github.com/dotnet/maui/issues/6961

### Public API Changes

None (maybe)

### Intended Use-Case

Show a platform UI for web view alerts and prompts.