Add @types/jest and simplify test example

[huntie]

Summary

This is now simpler, and more closely aligned with Expo's template. Also switch it() to test().

Changelog:
[General][Added] - Add @types/jest dev dependency, simplify test example

Test Plan

Apply changes to a freshly created 0.76.0-rc.2 project.

✅ @types/jest automatically discovered by TypeScript, allowing us to remove the old type import
✅ Tests pass

[cortinico]

@huntie can we have this in 0.77 instead?

[huntie]

Sure — this is main though. Let's create a 0.76-stable branch for this repo?

[cortinico]

Sure — this is main though. Let's create a 0.76-stable branch for this repo?

It's already there: https://github.com/react-native-community/template/tree/0.76-stable

[huntie]

Cool. I'm absolutely not suggesting this needs to be picked :)

[cortinico]

Cool. I'm absolutely not suggesting this needs to be picked :)

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