feat: Allow defining a custom class on the Modal's overlay

[pauloslund]

Short description

We need a way for consumers to define custom classes on the new Modal's overlay. This is needed to support custom styling for the overlay (eg. both the command menu, and quick add modal in Todoist), and for adding custom animations.

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
• [ ] 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

Patch

[pauloslund]

@gnapse Wanted to get your thoughts on this addition. I know generally we don't want to introduce exceptions to the design system, but I haven't been able to come up with an alternative approach that I liked. To clarify, some modals need to be able to restyle the included overlay. Generally this is by removing the background colour entirely, but they have also been adding classes to manage custom animations.