feat(cdk/scrolling): add input to enable append-only mode in virtual scroll viewport

[MichaelJamesParsons]

This PR adds an appendOnly input to cdk-virtual-scroll-viewport, which disables view recycling so items remain in the DOM even after they scroll out of view. This can be more performant for scroll viewports that render complex components that may be expensive to remove or insert at the beginning of the list while scrolling up.

Option questions

• Should the viewport throw if appendOnly changes after initialization? For example, if a user scrolls to the bottom of a list containing 2k items, then enables appendOnly, the viewport will render all 2k items.

[crisbeto]

It's been a while since I've looked at this code, but how is this different to setting the cache size to zero? I was under the impression that if a view can't be cached, it'll be recreated.

[mmalerba]

I think the difference is that with cache size zero, it won't reuse already created views, but it will remove them from the DOM. The idea with this change is to have a mode where it never removes anything from the DOM, only adds.

[MichaelJamesParsons]

+1

[MichaelJamesParsons]

I updated the description to clarify this behavior.

[mmalerba]

LGTM

[MichaelJamesParsons]

Updated the commit message to match the new PR title.

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