enhancement: offset item based on itemsize when using scrollToIndex method

I was running into issues where scrolling to an item index meant the item was not fully in the user's view (element was cut off). Updated `vue-virtual-scroller.esm.js ` below; not sure if the src code should be updated elsewhere but figured I would put this out for discussion at least.

```
scrollToItem (index) {
let scroll;
if (this.itemSize === null) {
scroll = index > 0 ? this.sizes[index - 1].accumulator : 0;
} else {
scroll = index * this.itemSize;
}
let { size } = this.sizes[index];
let offsetPosition = size ? scroll - size : scroll;
this.scrollToPosition(offsetPosition);
},