[feature]add current tag index interface

[wdjwxh]

Add a interface to get the current tag index in viewport.
Easy to get something from list

[sooxt98]

I just tested and it works for the first few item, but when i scroll further down, the index get offset further which means i need to scroll a little more to get the correct index

[jerrywell]

to whom needs this kind of function, please refer to this flutter/flutter#19941 (comment) for better performance. i'm closing it because the code is out of date for null safety version.