native, ui: move from TabStack to new custom NavBar component

[jamesacklin]

Renovates the app navigation to use nested Native Stacks and a custom NavBar component.

• Rather than using createBottomTabNavigator, uses createNativeStackNavigator for all nested "home" screens (ChatList, Channel, Post, ChannelSearch).
• Adds a custom navigation component (NavBar) in packages/ui which seeks to be as generic as possible
• Adds a NavBarView to the app screens themselves, which in turn takes the navigation prop from the parent screen and handles all the navigate() calls itself

Functionality is exactly the same as before but eliminates the gray box on view transition (fixes LAND-1873). Thanks to @patosullivan for the talk-through assist.

Note that this is an absolutely-positioned element, so any screen that uses a scroller that extends below the bottom edge of the screen will need to be padded up at the bottom.

[linear]

LAND-1873 native: gray box in place of tab bar when navigating into channel

[dnbrwstr]

Looks great! A few nitpicks.