fix(RecycleScroller): Introduce an item wrapper to reduce re-render

[ishitatsuyuki]

When rendering a slot, Vue will act as if the slot content was expanded in-line at the <slot>'s place.

Having the slots in a v-for therefore can trigger too much rerender for the slot content, and was generally bad for performance.

Wrapping them in a component should provide some performance improvements as well as solving the hover update overhead.

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