Add asDisposable convenience property to Task

Offers a convenient way to return Tasks as a `Disposable` instead of having to wrap it with `AnonymousDisposable`, for example in an `EffectHandler`

MobiusCore/Source/Disposables/Task+Disposable.swift

@@ -0,0 +1,7 @@
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
+public extension Task {
+
+    var asDisposable: any Disposable {
+        AnonymousDisposable { cancel() }
+    }
+}

MobiusCore/Test/Task+DisposableTests.swift

@@ -0,0 +1,29 @@
+import MobiusCore
+import Nimble
+import Quick
+
+@available(iOS 13.0, *)
+class TaskDisposableTests: QuickSpec {
+    override func spec() {
+        describe("Task+Disposable") {
+            var task: Task<Void, any Error>!
+            var disposable: Disposable!
+
+            beforeEach {
+                task = Task {
+                    try? await Task.sleep(nanoseconds: 1_000_000_000)
+                }
+                disposable = task.asDisposable
+            }
+
+            it("starts off not cancelled") {
+                expect(task.isCancelled).to(beFalse())
+            }
+
+            it("disposable cancels the task that owns it") {
+                disposable.dispose()
+                expect(task.isCancelled).to(beTrue())
+            }
+        }
+    }
+}