[Enhancement] Remove a specific Modal Page

### Description

I am testing the Modal Stack Navigation behavior.

- we are not able to remove a specific modal page,
The method RemovePage(Page page) does not remove modal pages

```csharp
Application.Current.MainPage.Navigation.RemovePage(this);
```

### Public API Changes

You can have another method called
````csharp
void INavigation.RemoveModalPage(Page page)
````

### Intended Use-Case

- A popup should be able to remove it self and only it self on close.
- We should be able to remove any modal page without removing the one on the top.
- Related to #11270

Start with the INavigation.RemovePage(Page page) entry point and the modal stack navigation behavior described in the issue. Determine how a specific modal page can be removed without removing the top page, including the proposed RemoveModalPage(Page page) API. Done means a popup can remove itself and callers can remove any selected modal page while preserving the remaining stack.

Written by the indexing model from the issue text.