iOS: Export RCTConvert+Transform.h header (#41179)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41179

Some libraries need to import it, e.g.:
https://github.com/software-mansion/react-native-svg/blob/f9c7d8a807c8dc5b21e8ac0358a9bf7a820a1f61/apple/ViewManagers/RNSVGNodeManager.mm#L12

But depending on the build configuration, `#import "RCTConvert.h"` may not be able to find the header properly, so let's move it to `<React/RCTConvert.h>`

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D50586352

fbshipit-source-id: a770008acec311a498cf667545bf899f9a8cbef9