Convert the editor part of the `<markdown>` component to Preact.

 - Remove the `readOnly` input as this mode is now handled by the
   separate `MarkdownView` component
 - Improve toolbar a11y by:
   - Adding the "toolbar" ARIA role and an aria-label to the toolbar container
   - Using `button` elements for toolbar buttons
   - Increasing the hit target area for buttons
 - Improve toolbar styling by:
   - Using flexbox
   - Using the modern color palette
   - Fixing the padding around the preview output