window scroll bindings are bidirectional

[Rich-Harris]

Ref #404. With this PR, in a situation like this...

<:Window bind:scrollY='y'/>

...changing the value of y programmatically will result in the window scrolling. (The value of y remains constrained to the scrollable range.)

In a lot of situations this kind of bidirectional binding is unnecessary, but it would violate expectations if it didn't work this way so I think it's worth the modest overhead, which basically amounts to this:

+var window_updating = false;
  
var onwindowscroll = function ( event ) {
+  window_updating = true;
  
  component.set({
    y: this.scrollY
  });
  
+  window_updating = false;
};
window.addEventListener( 'scroll', onwindowscroll );

+component.observe( 'y', function ( y ) {
+  if ( window_updating ) return;
+  window.scrollTo( window.scrollX, y );
+});

Combined with an animate or tween method (#236), this would provide a very straightforward way of smoothly scrolling to a section of the page, for example.