Install core-js to fix issue with compiled module (#18)

Since Babel is configured to inject core-js built-ins where needed, we
must also install the core-js package.

libraries/o-error/package.json

@@ -59,5 +59,7 @@
     "prettier": "^2.0.2",
     "typescript": "^3.8.3"
   },
-  "dependencies": {}
+  "dependencies": {
+    "core-js": "^3.8.3"
+  }
 }