Improves scroll interaction for sequence editor

- Scroll paning (`shift + scroll`) is now synced for the keyframe viewport, easing viewport and scrollbar (Closes #22)
-  Scroll zooming out (`ctrl + scroll down`) is now bounded to avoid zooming out to infinity  (Closes #19)

There is still some quirky behaviour when using scroll to zoom inwards. It does not seem to respect the `pivotPointInUnitSpace` correctly. I've tried fixing it, but I've hit a dead end.

packages/playground/src/index.html

@@ -7,6 +7,7 @@
         margin: 0;
         padding: 0;
         height: 100%;
+        background: black;
       }
     </style>
   </head>