Add back builder-webpack4 dep

Author: shilman

lib/core/package.json

@@ -40,10 +40,17 @@
     "@storybook/core-server": "6.2.0-alpha.29"
   },
   "peerDependencies": {
+    "@storybook/builder-webpack4": "6.2.0-alpha.29",
     "react": "^16.8.0 || ^17.0.0",
     "react-dom": "^16.8.0 || ^17.0.0"
   },
   "peerDependenciesMeta": {
+    "@storybook/builder-webpack4": {
+      "optional": true
+    },
+    "@storybook/builder-webpack5": {
+      "optional": true
+    },
     "typescript": {
       "optional": true
     }