Add PopupExtensions.ClosePopup()

[TheCodeTraveler]

Description of Change

This PR adds two additional extension methods to PopupExtensions.shared.cs:

public static Task ClosePopup(this INavigation, CancellationToken);
public static Task ClosePopup(this Page, CancellationToken);

After reading through @bijington's docs (amazing job, by the way!) , I realized that we don't give developers a way to close a Popup when they pass in a View (not passing in a Popup):

var label = new Label { Text = "Hello World" };
await this.ShowPopupAsync(label);
// How do I close the popup that `CommunityToolkit.Maui` auto-generated for the developer under the hood? 

With this PR, developers can now use PopupExtensions.Close():

var label = new Label { Text = "Hello World" };
await this.ShowPopupAsync(label);

await Task.Delay(TimeSpan.FromSeconds(2));

await this.ClosePopup();

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)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard

Additional information

This PR is targeting the #1581 PR. In other words, when this PR is merged, it will be merged into #1581, not the main branch.

[bijington]

LGTM! The only possible extra question I have is: should we add an example to the sample app that covers this functionality? Or was it already there and we just missed it during testing?

[TheCodeTraveler]

should we add an example to the sample app that covers this functionality

Good call! Done.