This updates the HTML for the formatting toolbar to ensure it's

accessible, enabling screen readers to understand the groups and items

on the toolbar in order to communicate it to the user, and to enable

keyboard navigation. This involves setting ARIA attributes and using

<button> instead of <a>.
Items are now registered dynamically instead of hard-coded. This is both

to ensure consistent HTML (particularly with accessibility) and to later

allow extensions to add buttons to the toolbar (which was an original

goal of the project).
This does make one minor change to the toolbar contents, which is to mov

the Code button alongside Bold/Italic/Strikethrough. This is because

it's just setting an inline style like those (it's a code literal, not a

code block), and has more in common with those than linking/uploading

media.

Tested all of the toolbar functionality. It all works (except for image

uploads, which is a known problem already).