Added more keyboard events to d3.event

-- these events come from https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent
   but I didn't include all of them

d3/d3-tests.ts

@@ -2656,4 +2656,18 @@ function multiTest() {
         .attr("class", "x axis")
         .attr("transform", "translate(0," + height + ")")
         .call(xAxis);
-}
+}
+
+// Tests miscellaneous keyboard events
+function keyboardEventsTest() {
+    var keyPressed: string;
+    d3.select("body").on("keydown", () => {
+        if (d3.event.metaKey) {
+            keyPressed = "meta";
+        } else if (d3.event.ctrlKey) {
+            keyPressed = "ctrl";
+        } else if (d3.event.altKey) {
+            keyPressed = "alt";
+        }
+    });
+}

d3/d3.d.ts

@@ -57,7 +57,9 @@ declare module D3 {
         x: number;
         y: number;
         keyCode: number;
-        altKey: any;
+        altKey?: boolean;
+        ctrlKey?: boolean;
+        metaKey?: boolean;
         type: string;
     }