feat: Add xsmall size to the Modal component

[rfgamaral]

Short description

This PR adds the xsmall size to the Modal component, and changes the small value from 450px to 480px so that the new Information/Confirmation modals can be stacked on top of other modals like this:

PR Checklist

• Updated docs (storybooks, readme)
• Executed npm run validate and made sure no errors / warnings were shown
• Described changes in CHANGELOG.md
• Bumped version in package.json and package-lock.json (npm --no-git-tag-version version <major|minor|patch>) ref
• Reviewed and approved Chromatic visual regression tests in CI

Versioning

Minor

[rfgamaral]

Re-rolling because Natalie is out today.