Extract the arrow key navigation logic from `MarkdownEditor` into a
reusable `useArrowKeyNavigation` navigation hook. This simplifies the
MarkdownEditor component and will allow us to enable arrow key
navigation more widely thoughout the application.

A notable design choice is that the roving tab index state lives in the DOM
rather than in Preact. This enables the Preact component-facing API to be very
simple: a single hook call in the component that renders the container element
(of the toolbar, menu bar etc.). It does mean however that the `tabIndex` state
is not accessible to components. This works for the use cases where I have
tested it, but we may need to revisit in future.

If browsers add a native feature to handle this in future [1], the hook
could handle testing for support and using it where available.

 - Create `useArrowKeyNavigation` navigation hook in
   `src/shared/keyboard-navigation.js` as a general method of adding
   arrow key navigation to a composite widget

 - Modify `MarkdownEditor` to use the `useArrowKeyNavigation` hook to
   handle arrow key navigation

 - Replace detailed tests for arrow key navigation in `MarkdownEditor`
   with more basic tests, as the `useArrowKeyNavigation` tests cover the
   general functionality in detail

[1] eg. https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Focusgroup/explainer.md