Implement `VerticalList` widget

Currently, people who want to create a new container widget look at the Flex widget for inspiration. That implementation is fairly complex, and not a great introduction for newcomers.

We should implement a container widget with a little code as possible. This widget would display its children vertically with a fixed amount of free space between them. It would have no customization parameters aside from that spacing constant. Its implementation would serve as a guide for people implementing custom widgets.

There should be non-doc comments in other widget files pointing newcomers to the VerticalList implementation.

To make sure the implementation doesn't bit-rot, we should:

• Include unit tests and screenshot tests.
• Use VerticalList in at least one example (probably to_do_list).
• Have a vertical list element is Xilem as well.

Start by reading the existing Flex widget and nearby widget implementations to understand the minimal container structure. Then inspect existing unit tests, screenshot tests, and the to_do_list example for patterns to follow. Done means VerticalList renders children vertically with fixed spacing, has Xilem support, is used in an example, includes both test types, and has newcomer pointers in other widget files.