jest: Also compile modules in @react-native, which we'll see with RN …

…v0.64.

This would be handled already if our transformIgnorePatterns
extended the transformIgnorePatterns in React Native's Jest preset,
because that preset was adapted in facebook/react-native@a77f2c40d,
released in RN v0.64.

It would also be handled already if our transformIgnorePatterns
extended the transformIgnorePatterns in `jest-expo`'s Jest preset
after *that* preset was adapted, in the (as-yet-unreleased) change
in expo/expo@24bce422c.

But we ignore and clobber transformIgnorePatterns in those presets,
because the API just isn't suitable to taking a list from elsewhere
and extending it [1]. So we add this line ourselves.

Since we're upgrading RN before taking that `jest-expo` change,
we'll see the following harmless warning for a while when we run
Jest:

  react-native/jest-preset contained different
    transformIgnorePatterns than expected.

It's harmless because we don't use `jest-expo`'s
transformIgnorePatterns anyway; we fully specify our own instead.

[1] zulip#4991 (comment)

Related: zulip#4426

jest.config.js

@@ -7,6 +7,7 @@ const transformModulesWhitelist = [
   'expo-application',
   'expo-web-browser',
   'react-native',
+  '@react-native',
   // @rnc/async-storage itself is precompiled, but its mock-helper is not
   '@react-native-async-storage/async-storage',
   '@react-native-community/cameraroll',