breaking: New notification toasts, deprecate and rename legacy Notification

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Short description

These new components should be able to replace all our similarly-purposed components that we have in both Twist and Todoist with more or less repeated concerns, and currently not sharing a single implementation.

I plan to give it a go locally in our apps first (that is, running Twist or Todoist locally with this version of Reactist also locally) and see if I can replace our in-app toast-like components with these new components, before even considering merging this. But it may be valuable that we start the review of this early on as well.

• Twist Discussion
• Figma ref

Demo

Toast	Notifications

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

This will require a new major version, due to the renaming of the legacy Notification component as DeprecatedNotification.