Upgrade Storybook and set up accessibility testing addon

[doubleedesign]

The main purpose of this PR is to introduce the accessibility (a11y) testing add-on into our Storybook workflow. As part of this, I:

• Refactored the theme wrapping decorator into its own component and file, due to the need to add a11y-addon related code to the decorator(s) as well.
• Created a new, separate WithA11yTests decorator to add a workaround for a bug where the a11y test results come back empty when changing the selected story (seems to be a timing issue, race condition, or something along those lines). How this works is when a story renders/re-renders and the a11y tests run, if the three result categories (passes, violations, and incomplete) all return empty, a re-run of the test is triggered.
• Upgraded Storybook to the latest version, because that's just good dependency hygiene.
• Separated and updated some ESLint config because it wasn't working correctly on Storybook config files.
• Fixed some miscellaneous TypeScript warnings that I came across in the course of this work.