fix: v2 item separator with inverted flashlist

[theeket]

Description

When inverted is true, the ItemSeparatorComponent should be rendered before the item (vertical: top, horizontal: left) to make it render properly. Otherwise, the separator will be displayed at the wrong side.

This fix once landed on v1.6.4. However, the same behavior does not being carried over to v2.
#638, PR #1048

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Screenshots or videos (if needed)

Example: inverted, horizontal

In the “Before” picture, the separator displayed at the right, so the last item (at the left) which doesn’t have a separator will displayed adjacently and the first item (at the right) will have extra space. In “After” picture, the separator displayed at the left, makes it correct.

Before:

---

After: