fix: re-export AssetRegistry from the react-native components shim

[oJaqob]

What

Adds AssetRegistry to the react-native-css/components re-exports.

Why

The Metro resolver sends react-native imports from node_modules to this file. React Native 0.88 added AssetRegistry to its public exports, and expo-asset does import { AssetRegistry } from 'react-native'. Without this getter, expo-asset receives undefined and every asset load fails:

TypeError: Cannot read property 'getAssetByID' of undefined
    at fromModule (expo-asset/build/Asset.js)

In an app this shows up as useFonts failing and the app silently falling back to system fonts.

I compared the export list of react-native/index.js (0.88.0) with this file. AssetRegistry was the only name missing.

Checked

• Reproduced on Android with react-native 0.88.0, expo 58.0.13 and expo-asset 58.0.16: fonts fell back to the system font before the change and load correctly after it.
• I haven't run the repo's own test suite.