[scrollable_positioned_list] Added ItemScrollController.jumpToPixel and .currentScrollControllerOffset

[TheCarpetMerchant]

Description

Added a method to call primary.scrollController.jumpTo and a getter for primary.scrollController.offset.
I used the null-safe operator ! for _scrollableListState without checking for null given that is how it is done in other methods and the existence of isAttached.

Open to suggestions about naming, I don't think the current names are very good but couldn't come up with anything better.

Please note this is my first PR ; be sure to comment about any mistakes I might have made in the process.

Edit : Added a second commit for scrollToPixel, which allows calling primary.scrollController.animateTo.

Related Issues

No related issues, this is just something I need that might be useful for others too.

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