This PR does a couple of things!
https://user-images.githubusercontent.com/16964204/231897483-416287f9-50ce-468d-a714-2a4bc0f2e011.mov
Fixes#20819Fixes#41910Fixes#121419
### Adds ScrollController.onAttach and ScrollController.onDetach
This resolves a long held pain point for developers. When using a scroll controller, there is not scroll position until the scrollable widget is built, and almost all methods of notification are only triggered when scrolling happens. Adding these two methods will help developers gain access to the scroll position when it is created. A common workaround for this was using a post frame callback to access controller.position after the first frame, but this is ripe for issues such as having multiple positions attached to the controller, or the scrollable no longer existing after that post frame callback. I think this can also be helpful for folks to debug cases when the scroll controller has multiple positions attached.
In particular, this also resolves this commented case: https://github.com/flutter/flutter/issues/20819#issuecomment-417784218
The isScrollingNotifier is hard for developers to access.
### Docs & samples
I was surprised we did not have samples on scroll notification or scroll controller, so I overhauled it and added a lot of docs on all the different ways to access scrolling information, when it is available and how they differ.
This does a cleanup of the examples, removing all of the "section" markers and extra comments that we don't need anymore now that the samples are no longer in the source code. It also removes the --template arguments from the {@tool dartpad} and {@tool sample} directives, since those are no longer used. It converts two examples that I discovered were still embedded into linked examples in the examples folder.
I didn't delete the templates from the snippets config folder yet, because there are still embedded samples in the dart:ui package from the engine that use them. Once dart:ui no longer uses the templates, they can be removed.
I bumped the version of the snippets package to pick up a change that allows removal of the --template argument.
This extracts the sample code out from the API doc comments, and places them in separate files on disk, allowing running of the examples locally, testing them, and building of slightly larger examples.
