anchors 1/n: Handle GrowthDirection.reverse in sticky_header

[gnprice]

This is the first of several PRs leading us to #80 and #82, opening the message list at an anchor other than the latest message in history.

For this PR, the main work is to lift one of the simplifying assumptions I originally made for an MVP of the sticky_header library, allowing the SliverStickyHeaderList to be a sliver that counts backward in the enclosing ScrollView rather than forward.

We also rearrange some things in how MessageList builds the list, in order to exercise that new flexibility and to get closer to how we'll want things when implementing #80 and #82.

Main commit messages

sticky_header: Handle GrowthDirection.reverse

This resolves the last of the "TODO dir" comments in this library.

We had originally written this library to rely on the assumption
that constraints.growthDirection was GrowthDirection.forward,
i.e. that the direction in which constraints.scrollOffset increases
was the same as constraints.axisDirection, in order to keep down
the number of distinct directions one needs to keep track of in
writing and reading (and debugging) the implementation.

But now that this logic is relatively solid within that case --
and now that we have a solid test suite for this library -- we can
go back and identify the handful of places that need to be flipped
around for GrowthDirection.reverse, and do so.

This support isn't yet especially useful: there are a couple of
unrelated bugs which are triggered when the area allotted to this
sliver isn't the entire viewport, and there isn't a lot of reason
to have a sliver with GrowthDirection.reverse when no other sliver
is potentially occupying part of the viewport. We'll fix those
separately.

---

msglist [nfc]: Let ScrollView count forward, and sliver handle reverse

This means that the overall scroll view has AxisDirection.down
as the scroll direction, rather than AxisDirection.up, and the
SliverStickyHeader is passed GrowthDirection.reverse instead
of GrowthDirection.forward in order to get the same effect.

This makes the scrolling a little easier to think about when focused
on MessageList's code, because for example (as seen in this diff)
scrollMetrics.extentBefore now refers to the messages that are
older than the ones on screen, rather than those that are newer.

This also brings us closer to the setup we'll want for #80 and #82,
opening the message list at an anchor other than the end: the empty
placeholder sliver at the bottom will become the list of messages
after the starting anchor, while the sliver at the top will remain
the list of messages above the anchor.

[chrisbobbe]

Thanks! Merged.