Implement backBehavior

[okwasniewski]

Native Bottom Tabs are missing this prop from JS Tabs, this can be replicated almost 1:1 as it lays on the JS side:

backBehavior

This controls what happens when goBack is called in the navigator. This includes pressing the device's back button or back gesture on Android.

It supports the following values:

• firstRoute - return to the first screen defined in the navigator (default)
• initialRoute - return to initial screen passed in initialRouteName prop, if not passed, defaults to the first screen
• order - return to screen defined before the focused screen
• history - return to last visited screen in the navigator; if the same screen is visited multiple times, the older entries are dropped from the history
• none - do not handle back button