Move types to @amplitude/experiment-core

Hi team,

Would it be possible to merge duplicate core types such as `client.d.ts`, `variant.d.ts`, and other types from these two packages:
- `@amplitude/experiment-react-native-client/lib/typescript/types/*`
- `@amplitude/experiment-js-client/dist/types/src/types/*`

... into `@amplitude/experiment-core` ?

Typing the ExperimentClient either from the Web type **OR** the React Native type seems like a bit of a shame when it seems they're pretty much the same?

I'm implementing a Provider that's meant to be common, both to our React web app + and our two React Native apps, both using typescript on a monorepo. My Provider is supposed to receive an `ExperimentClient`, either instantiated from `@amplitude/experiment-react-native-client` or `@amplitude/experiment-js-client` 🙏