feat(cdk/scrolling): ScrollToIndexIfNeeded function

#### Feature Description

We have `scrollToIndex` function on `CdkVirtualScrollViewport`. But we cannot determine whenever an index is or is not withing **visible** viewport.

```ts
const viewPort: CdkVirtualScrollViewport;

scrollToIndex(index: number): void {
const range = viewPort.getRenderedRange();

if (range.start > index || range.end < index) {
// Scroll is 100% needed.
viewPort.scrollToIndex(index, 'auto');
} else {
// The index is within range, but not necessarily within visible viewport!
// From my testing I have +-8 items above and bellow at the time
}
}
```

#### Use Case

It would enable scrolling to an item if only needed.

#### Note

If there was `CdkVirtualScrollViewport.getVisibleRange`, we could even scroll our desired element to the middle of the viewport!

Start with CdkVirtualScrollViewport, especially the existing scrollToIndex and getRenderedRange entry points. Determine how a visible range could be exposed or used to avoid unnecessary scrolling, including the requested middle-of-viewport use case. Done means the viewport can scroll to an index only when it is outside the visible viewport.

Written by the indexing model from the issue text.