Add `align_self` property

We already support `align_items`, which aligns all of a box's children at once in the cross axis. We should also support `align_self`, which allows the alignment to be overridden on a per-child basis.

To avoid confusion, the existing alignment-related bugs should probably be fixed first (#2213).

Based on #3111, this property should have the alias `horizontal_align` or `vertical_align` when the parent is a column or row respectively. But this is complicated by the fact that during initialization, we don’t know what the parent will be yet.

Start by reviewing the existing align_items support and the alignment-related bugs in issue #2213, then read issue #3111 for the proposed aliases. Done means align_self can override alignment for an individual child and the parent-dependent alias behavior has a settled, implemented design.

Written by the indexing model from the issue text.