Update "React Native Tools" VSCode extension

[afoxman]

After getting proposal approved (#1274), write the code to implement it.

Draft PR in internal LPC repo is a starting-point. Add platform selector and mapping strategies. Add callout to update tsserver when platform changes. This may require changes to TypeScript extension or VSCode itself (hopefully not).

Add unit tests if a framework is available.

Verify manually, looking at import/require resolutions shown on hover.