feat: Support additional custom offset in case of pinned header above…

[Ortes]

No breaking change (I think)
Add support for addition custom offset.

.scrollToIndex() now takes an additional nullable offset parameter which move the final offset by this amount.

[jerrywell]

Hi @Ortes,

This feature can actually be achieved using an alternative method.

e.g.

await scrollController.scrollToIndex(10);
await scrollController.animateTo(scrollController.offset + [Offset], ...);

since the underlying implementation works the same way. : )

[Ortes]

Indeed it works but it does not have the exact same behavior because it scroll to the offset then backward (I put negative values because I have a pinned header above so I need to offset negatively)