Ensure CI creates a chromatic baseline against main (#2756)

Author: carbonrobot

.github/workflows/chromatic.yml

@@ -1,8 +1,12 @@
 name: Chromatic
 
 # Runs chromatic on:
-#   every pull request where there are changes in stories/ or packages/ and the PR is not in draft mode
+# - every push to main (to create a chromatic baseline)
+# - every pull request where there are changes in stories/ or packages/ and the PR is not in draft mode
 on:
+  push:
+    branches:
+      - main
   pull_request:
     branches:
       - main