[0.9.4] - Release Candidate 2

📜 Curtains

[0.9.4] - Release Candidate 2

👥 package:shadows forked off.

Added two 🆕 fields for initialization:

1. endCurtainInitVisible
2. alignment

Both have fairly descriptive names:

1. In the past, endCurtain always started visible, regardless of scrollable child's content. (startCurtain always began and still begins hidden.) This is due to _Curtain appearance logic being controlled by a ScrollUpdateNotification listener versus a ScrollController, and a Curtains has no knowledge of the scrollable until Notifications are received by user scroll.
2. Alignment is handled by Curtains considering scroll direction and text direction, but it may now be overridden.

One 🐞 bug remains:

1. Even with Curtains.spread set non-negligibly, BoxDecoration.backgroundBlendMode does not work correctly.