CI: require changesets before merging into version-4 (#6787)

Changes that don't touch packages don't require changesets.

If you made a change to a package that you don't want to show up as a
changeset, just run `changeset --empty` like the error says.

(This does assume that your remote is named `origin`. We pass
`origin/version-4` instead of `version-4` because that way CI doesn't
have to actually create a local `version-4` branch. We don't set
`baseBranch` to `origin/version-4` in config.json because enforcing the
remote name even on `changeset add` users seems bad.)

.circleci/config.yml

@@ -65,6 +65,13 @@ jobs:
  - setup-node
  - run: npm run lint
 
+ Changesets:
+ docker:
+ - image: cimg/base:stable
+ steps:
+ - setup-node
+ - run: npm run changeset-check
+
  Spell check:
  docker:
  - image: cimg/base:stable
@@ -106,3 +113,4 @@ workflows:
  - ESLint
  - Spell check
  - Smoke test built package
+ - Changesets

package.json

@@ -30,7 +30,8 @@
  "prettier-check": "prettier --check .",
  "prettier-fix": "prettier --write .",
  "spell-check": "cspell lint '**' --no-progress || (echo 'Add any real words to cspell-dict.txt.'; exit 1)",
- "changeset-publish": "changeset publish"
+ "changeset-publish": "changeset publish",
+ "changeset-check": "changeset status --verbose --since=origin/version-4"
  },
  "workspaces": [
  "packages/*"