Breaking: Deprecate Reach-UI based modal and add Ariakit-based modal

[frankieyan]

Closes https://github.com/Doist/reactist/issues/679

Short description

This re-implements the modal with Ariakit. It tries to match the previous implementation as close as possible, both API and markup wise, so we don't leverage Ariakit's default modal backdrop.

Test plan

Test that all of the previous modal's functionality have been retained in the storybook demos.

Old: http://localhost:6006/?path=/docs/components-modal New: http://localhost:6006/?path=/story/design-system-modal

• [x] The modal can be closed by clicking on the backdrop or pressing Esc
• [x] Right clicking on the backdrop doesn't close the modal
• [x] The modal isn't rendered in the DOM when it's closed
• [x] Other elements that are siblings to the modal have aria-hidden set when the modal is open
• [x] Content outside the modal isn't scrollable when it's open
• [x] The modal's contents should be scrollable
• [x] The width/size props should be respected

PR Checklist

• [x] Added tests for bugs / new features
• [x] Updated docs (storybooks, readme)
• [x] Executed npm run validate and made sure no errors / warnings were shown
• [x] Described changes in CHANGELOG.md
• [ ] Bumped version in package.json and package-lock.json (npm --no-git-tag-version version <major|minor|patch>) ref
• [ ] Updated all static build artifacts (npm run build-all)

Versioning

Major