When content size inside the before slot changes, virtual scroller breaks

The size of the `before` slot is not being watched for changes and the virtual scroller breaks whenever such content changes. You can see the issue on this CodeSandbox:
https://codesandbox.io/embed/vue-virtual-scroller-issue-x22rm

**Steps to reproduce:**
1. Open the CodeSandbox.
2. Try to scroll in the demo, everything will work as expected.
3. Click the red button labeled "Click to break" at the top of the page.
4. Scroll the page again. The items in the list will disappear before intended.

The expected would be that if the `before` slot changes, the calculations that vue-virtual-scroller does are also updated. In theory, it can be accomplished by using mutation observers on the `before` and `after` slots.