Provide alternative for reordering items without dragging with mouse

Currently there is no keyboard accessible alternative for reordering the task items which prevents assistive technology/ keyboard-only users from accessing the feature.

A number of implementations I've seen like [this example from salesforce-ux][1] enable user to press `SPACE` to move an item around with arrow keys and `SPACE` to place an item. Something to keep in mind for this element is making sure to not interrupt checking of checkbox (which can also be done `SPACE`).

Other references:

[Upcoming WCAG change 2.5.7 Dragging Movements (AA)](https://www.w3.org/WAI/standards-guidelines/wcag/new-in-22/#257-dragging-movements-aa)

[4 major patterns for accessible drag and drop](https://medium.com/salesforce-ux/4-major-patterns-for-accessible-drag-and-drop-1d43f64ebf09)

[1]: https://salesforce-ux.github.io/dnd-a11y-patterns/#/sortB?_k=1lp55f

No file or test is named in the issue. Start by locating the task-list drag-and-drop and item-reordering entry points, then review how keyboard focus and checkbox activation work. Done means keyboard-only and assistive-technology users can reorder items without dragging, while SPACE still supports checkbox interaction.

索引モデルが issue の本文から書いたものです。