Provide an extension method to detect if popup was previous page

[bijington]

Description of Change

This introduces the ability for developers to determine whether the previous page displayed in their app was a PopupPage as below

public class MyPage : Page
{
    protected override void OnNavigatedTo(NavigatedToEventArgs args)
    {
        base.OnNavigatedTo(args);

        if (args.WasPreviousPageAToolkitPopup() is true)
        {
            return;
        }

        // do other stuff
    }
}

The reason for the extension method over allowing a developer to just check the PreviousPage property is twofold:

1. PreviousPage is currently internal
2. PopupPage is also internal

I understand that internal access to MAUI will be removed from MAUI for .NET 10.0 but it also appears that this will be introduced dotnet/maui#28384. If that is the case the code in this PR will continue to function when .NET 10.0 ships.

Linked Issues

• Fixes #

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description) - the constructor for NavigatedToEventArgs is internal so it would require some reflection to construct something to test.
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

[Copilot]

Pull Request Overview

Adds an extension method to NavigatedToEventArgs that allows developers to detect if the previous page was a Toolkit popup.

• Introduces WasPreviousPageAToolkitPopup extension.
• Checks args.PreviousPage is PopupPage.
• Adds new file under Extensions namespace.

Comments suppressed due to low confidence (2)

src/CommunityToolkit.Maui/Extensions/NavigatedToEventArgsExtensions.shared.cs:11

• The XML comment references but the method checks for PopupPage; update the cref to for consistency.

	/// Determines whether the previous page was a Community Toolkit <see cref="Popup"/>.

src/CommunityToolkit.Maui/Extensions/NavigatedToEventArgsExtensions.shared.cs:15

• Consider adding unit tests covering the cases where the previous page is a PopupPage and when it's not to ensure the extension behaves correctly.

	public static bool WasPreviousPageAToolkitPopup(this NavigatedToEventArgs args) => args.PreviousPage is PopupPage;

[bijington]

Thanks for the approval @pictos

@jfversluis do you have any understanding on whether dotnet/maui#28384 will make it into .NET 10.0?

[jfversluis]

Asked @PureWeen or maybe @jsuarezruiz knows