feat: add origin prop for <transition-group>, fix #8424

[Justineo]

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• Bugfix
• Feature
• Code style update
• Refactor
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• Yes
• No

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As described in #8424, <transition-group> now records the items' position relative to the viewport (via getBoundingClientRect()). While page scroll happens just before the items start to perform transition, it will look like that they are flashed out of the container and slowly move to their original position.

This PR added a new prop: origin for <transition-group>, which is a string value that indicates how we are gonna calculate the position of transition items. Available values are viewport (the current behavior) and document. When set to document, the position of the items are always calculated relative to the top left corner of the document element, which will retain their position in the document when page scroll occurs.

[Justineo]

On second thought: we are not always performing page scroll on the root element. If the page scroll is triggered by an element other than the root element, this fix would fail. Perhaps we should provide a way for users to specify the “origin” element themselves?

[tmorehouse]

What about detecting the scrollParent of the element? walking up the DOM tree to find out which element is scrollable.

[Justineo]

@tmorehouse The problem is that scroll doesn't have to be triggered on the closest scrollParent. We may have enum values like viewport/document/scrollParent or find a way to let users specify the origin manually.