Story rework: VaVirtualScroller

[xiongmao86]

Transform VaVirtualScroller to storybook.
Part of #3676.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

[xiongmao86]

Hi, I have most props test by step in storybook. But there are two issue I don't know how to handle:

1. track-by prop is assign to key attribute, which is handle and remove by vue, and won't show up in the DOM of browser, how can I test for it?
2. In the story of DifferentSizesAndMargins the bottom padding and button size is different in even and odd list items, I am not sure how to write test for it, I don't know how the size are calculated, I can work it out by reading the code, but I am not sure whether that is a good way, would it be rigid and not flex?

Is there anything I have missed to do?

[xiongmao86]

Is it normal for UI Tests to fail? I haven't see anything related to VaVirtualScroller in details?

[asvae]

Hey, great work!

There is a bit or redundancy in old demos and this PR, apart from that - things are perfect.

[asvae]

Is it normal for UI Tests to fail? I haven't see anything related to VaVirtualScroller in details?

Generally no (tests pass on develop). But you can ignore that for this PR 😁 as they have nothing to do with your changes (likely tests are broken because you branched off a while ago).

[xiongmao86]

Thanks, @asvae, How about the code now, anything I have missed?

[asvae]

I did a couple of small changes, but overall it's perfect 🤗.

[xiongmao86]

Thank you very much, @asvae.