`NestedScrollView`'s outer scrollable jumping with `BouncingScrollPhysics` due to `double` precision errors (#138319)

This PR fixes scrolling issues with `NestedScrollView` using the `BouncingScrollPhysics`. In one of the steps of the calculation, we can reach a state where the position of the inner scrollable is set to a `double` value that falls within `precisionErrorTolerance` of `0` but we were using `==` with `0` rather than checking for a precision. My posts in the linked issue show the current behavior, and how I reached to conclusion (the code in this PR). This PR only addresses the "jumping" of the outer scrollable.  

Fixes #136199

I have not finished a test for this since I have never done so and therefore have 0 experience writing tests in Flutter, so any help there would be appreciated. I am also not sure how to test double precision errors in general. I did run all the nested_scroll_view_tests.dart locally and there are no failures.