Fix Skia-Reanimated integration (#5103)

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## Summary

This PR fixes broken integration with Skia. The concept is to export
publicly `startMapper` and `stopMapper` for now but in the future
reexport those functions with different names so they would be more
explicit about what they do.

## Test plan

Check if Skia works.

src/reanimated2/index.ts

@@ -259,3 +259,4 @@ export type {
 } from './helperTypes';
 export type { AnimatedScrollViewProps } from './component/ScrollView';
 export type { FlatListPropsWithLayout } from './component/FlatList';
+export { startMapper, stopMapper } from './mappers';