Remove the package access modifier and update the store's interface

[DevYeom]

Related Issues 💭

• The package acess level used on '...' requires a package name; #98

Description 📝

• Made the isIdle property of Store public.
• The expect function is only supported in Store.
• Updated the Testing documentation.

Additional Notes 📚

let sut = Store(
    reducer: TestReducer(),
    state: TestReducer.State(count: 0)
)
await sut.send(.increment)
await sut.expect(\.count, 1)

Checklist ✅

• If it's a new feature, have appropriate unit tests been added?
• If the changes affect existing functionality, please verify whether the above information has been appropriately described.