Split out Jest preset to @react-native/jest-preset

[kitten]

Summary:

This implements the proposal to split the Jest preset (and dependencies) out into a separate package: react-native-community/discussions-and-proposals#926

• react-native has an optional peer dependency on @react-native/jest-preset
• react-native/jest-preset.js throws when @react-native/jest-preset isn't installed and instructs the user to add it to migrate
• @react-native/jest-preset may be used directly as a Jest preset

Changelog:

[GENERAL] [BREAKING] - Move Jest preset to new @react-native/jest-preset package

Test Plan:

The jest preset is used, with some modifications, in facebook/react-native itself and CI should confirm that the changes work. Running jest on the repo similarly validates that the preset still works.

Otherwise, nothing has changed and the easiest way to test this is to use the preset in an existing project and try both preset set to the current value and preset set to "@react-native/jest-preset"

[huntie]

Thanks for this great README ❤️

[huntie]

quick nit: Can we use packages/jest-preset/ as the dir name? (I appreciate we aren't consistently one way or the other yet!)