Implements the `.value` async property for `Future` available since iOS 15.0

Future+Async.swift

import Combine

extension Publishers {
    struct MissingOutputError : Error {}
}

extension Future where Failure == Never {
    var value: Output {
        get async {
            var cancellable: AnyCancellable? = nil
            var receivedValue: Output? = nil
            
            return await withCheckedContinuation { continuation in
                cancellable = sink { completion in
                    switch completion {
                    case .failure(let error):
                        continuation.resume(throwing: error)
                    case .finished:
                        if let receivedValue = receivedValue {
                            continuation.resume(returning: receivedValue)
                        }
                    }
                    
                    cancellable?.cancel()
                } receiveValue: { value in
                    receivedValue = value
                }
            }
        }
    }
}

extension Future {
    var value: Output {
        get async throws {
            var cancellable: AnyCancellable? = nil
            var receivedValue: Output? = nil
            
            return try await withCheckedThrowingContinuation { continuation in
                cancellable = sink { completion in
                    switch completion {
                    case .failure(let error):
                        continuation.resume(throwing: error)
                    case .finished:
                        if let receivedValue = receivedValue {
                            continuation.resume(returning: receivedValue)
                        }
                    }
                    
                    cancellable?.cancel()
                } receiveValue: { value in
                    receivedValue = value
                }
            }
        }
    }
}