[knockout] Introduce specialized signatures for subscribe method, specifically common "change" and "beforeChange"

Add subscribe for "arrayChange" event for observable array (fixes #11203)
Improve KnockoutArrayChange status typing.

knockout/tests/knockout-tests.ts

@@ -680,3 +680,38 @@ function test_tasks() {
         setTimeout(callback, 0);
     };
 }
+
+function observableEventsTests() {
+    var observable = ko.observable(1);
+    observable.subscribe(value => {
+        var num: number = value;
+    });
+    observable.subscribe(value => {
+        var num: number = value;
+    }, null, "change");
+    observable.subscribe(value => {
+        var num: number = value;
+    }, null, "beforeChange");
+}
+
+function observableArrayEventsTests() {
+    var observableArray = ko.observableArray([1, 2, 3, 4]);
+    observableArray.subscribe(array => {
+        var arr: number[] = array;
+    });
+    observableArray.subscribe(array => {
+        var arr: number[] = array;
+    }, null, "change");
+    observableArray.subscribe(array => {
+        var arr: number[] = array;
+    }, null, "beforeChange");
+    var count = 0;
+    observableArray.subscribe(changes => {
+        changes.forEach(change => {
+            if (change.status == "added")
+                count++;
+            else if (change.status == "deleted")
+                count--;
+        });
+    }, null, "arrayChange");
+}