dillon1000/react

[babel] Alias unstable_useMemoCache to useMemoCache

DevTools relies on built-in hook names at their call site to be unprefixed in 

order to correctly track them. This PR updates our Babel plugin to: 

- Check if there are any existing import declarations of `import { /* ... /* } 
from 'react';` 

- If true, we add the specifier `unstable_useMemoCache as useMemoCache` 

- Otherwise, we synthesize a new import declaration 

- In Codegen we now emit `useMemoCache(n)` rather than 
`React.unstable_useMemoCache(n)`