Mark prosemirror.css file as having side-effects in package.json

FIX: Mark prosemirror.css as having side effects in package.json.

Author: vmagamedov

package.json

@@ -13,7 +13,7 @@
     },
     "./style/prosemirror.css": "./style/prosemirror.css"
   },
-  "sideEffects": false,
+  "sideEffects": ["./style/prosemirror.css"],
   "style": "style/prosemirror.css",
   "license": "MIT",
   "maintainers": [