Added extension, animateTo, clickIndex

[YDA93]

Similar to https://pub.dev/packages/convex_bottom_bar

I have made 4 useful updates:

1. animteTo which will only change the active index without any firing tab events.
2. clickTo which will fire click event to wanted index similar to a manual click.
3. Made the state public so we can make keys to control the nav bar.
4. Added extensions so now you can do the following:

context.gNav?.animateTo(tabsRouter.activeIndex);
context.gNav?.clickIndex(tabsRouter.activeIndex);

or through using keys:

appBarKey.currentState?.animateTo(tabsRouter.activeIndex),
appBarKey.currentState?.clickIndex(tabsRouter.activeIndex),