Make scroll handling contextual (#2200)

* Add an intermediate List component

* Fix type

* Add onScrolledDownChange

* Port pager to use onScrolledDownChange

* Fix on mobile

* Don't pass down onScroll (replacement TBD)

* Remove resetMainScroll

* Replace onMainScroll with MainScrollProvider

* Hook ScrollProvider to pager

* Fix the remaining special case

* Optimize a bit

* Enforce that onScroll cannot be passed

* Keep value updated even if no handler

* Also memo it

src/view/com/util/Views.d.ts

@@ -1,6 +1,6 @@
 import React from 'react'
 import {ViewProps} from 'react-native'
-export {FlatList, ScrollView} from 'react-native'
+export {FlatList as FlatList_INTERNAL, ScrollView} from 'react-native'
 export function CenteredView({
   style,
   sideBorders,