Add Create effect that creates an asynchronous stream

[DevYeom]

Related Issues 💭

Resolves #69

Description 📝

• Add Create effect that creates an asynchronous stream.
• Add unit tests for Create effect.

Additional Notes 📚

func reduce(state: inout State, action: Action) -> AnyEffect<Action> {
    switch action {
// ...
        case .twice:
            return .create { continuation in
                continuation.yield(.increment)
                continuation.yield(.increment)
                continuation.finish()
            }
// ...
    }
}

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.

[DevYeom]

@sobabear

It would be good to refer to the test_createAsynchronouslyWithCompletionHandler case.

[sobabear]

@DevYeom Thank you for your quick fixs!
To be honest, create syntax is what I wanted mostly. Very brilliant decision!