add getter for atRoot

[dickermoshe]

What feature would you like to see?

The current implementation of WoltModalSheets navigator is really fantastic.
It would be even better if there was a way to access how many pages are in the stack.

This is relevant for making a dynamic back button for a page.
If we knew that we were at index 0, it would show a close button that closes the sheet, or a back button that navigates back.

_currentPageIndex is private now, it would be awesome if it wasn't.

[ulusoyca]

@dickermoshe Thanks! Our conversation was really helpful. Would you consider opening a PR?

[dickermoshe]

#204

[dickermoshe]

My only complaint is that now WoltModalSheet... is polluted when looking for the show function.

Like, these aren't really that helpful, the code is basically the same.

Flutter uses .of for looking up controllers in context, that's what users are used to
I would've kept it all hidden behind .of()

This is just my humble opinion, otherwise the implementation is top notch

[ulusoyca]

Thank you for the feedback! Yes, makes sense. This PR removes the duplicate methods and sticks with WoltModalSheet.of(context).method type calls.