Add changesets

[ianstormtaylor]

Description

This adds 🦋 Changesets to make managing releases easier. It adds a few GitHub actions that will automatically release everything on master to the dev tag on NPM. And it will automatically make pull requests that can be merged to release to the latest tag too.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)

[changeset-bot]

✅ Changeset detected. 2af1f4f

The changes in this PR will be included in the next version bump.

Package	Type
slate	major	🔴
slate-history	patch	🟢
slate-hyperscript	minor	🟡
slate-react	patch	🟢
slate-test-utils	—	⚪️

• Not sure what this means? Click here to learn what changesets are.
• Want to add a changeset? Click here to add another changeset to this PR.