modify: fix OverlayEntry can't be removed correctly while window size changed

[Switernal]

I found a problem: if I change window size on destop, the previous OverlayEntry can't be removed from Navigator.of(context).overlay correctly.

I checked and found the problem is that while window size changed, the InAppNotification will be rebuilt, and then the _VsyncProvider is rebuilt. However, the build() method of __VsyncProviderState returns a _NotificationController(), where the _NotificationState is created.

Which means, while window size changed, the previous _NotificationState is missed after InAppNotification rebuilt , with the OverlayEntry missed.

So I choose to save _NotificationController in __VsyncProviderState. Before _VsyncProvider is rebuilt, the previous OverlayEntry can removed correctly.

Also, I confirm that this package can work well with flutter 3.10.6 and dart 3.0.6, so I changed dart SDK version to '<4.0.0'.

[github-actions]

LCOV of commit 49388b6 during Testing #37

Summary coverage rate:
  lines......: 68.7% (134 of 195 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate:
                                            |Lines       |Functions  |Branches    
  Filename                                  |Rate     Num|Rate    Num|Rate     Num
  ================================================================================
  lib/src/vsync_provider.dart               | 100%     20|    -     0|    -      0

[github-actions]

LCOV of commit af9c7db during Testing #38

Summary coverage rate:
  lines......: 68.7% (134 of 195 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate:
                                            |Lines       |Functions  |Branches    
  Filename                                  |Rate     Num|Rate    Num|Rate     Num
  ================================================================================
  lib/src/vsync_provider.dart               | 100%     20|    -     0|    -      0