feat(VVirtualScroll): add scrollToIndex options and fractional scroll

[koush]

Description

scrollToIndex does not scroll to fractional values. Fractional values would allow scrolling to a point between two elements.

For example, scrollToIndex(1.5) should scroll halfway between indices 1 and 2.

scrollToIndex also does not support scroll options, which makes it difficult to programmatically use smooth vs instant scrolling.

This change should be backwards compatible with existing consumers of this API:

• fractional values did not work at all prior to this change, and would fail.
• the second options argument did not exist.