fix: add missing dependency to package.json (#3753)

webpack-sources is used in code but not declared in package.json

ref https://github.com/pnpm/pnpm/issues/891

package.json

@@ -105,6 +105,7 @@
     "webpack": "3.10.0",
     "webpack-dev-middleware": "1.12.0",
     "webpack-hot-middleware": "2.21.0",
+    "webpack-sources": "1.1.0",
     "write-file-webpack-plugin": "4.2.0",
     "xss-filters": "1.2.7"
   },

yarn.lock

@@ -6882,7 +6882,7 @@ webpack-hot-middleware@2.21.0:
     querystring "^0.2.0"
     strip-ansi "^3.0.0"
 
-webpack-sources@^1.0.1, webpack-sources@^1.1.0:
+webpack-sources@1.1.0, webpack-sources@^1.0.1, webpack-sources@^1.1.0:
   version "1.1.0"
   resolved "https://registry.yarnpkg.com/webpack-sources/-/webpack-sources-1.1.0.tgz#a101ebae59d6507354d71d8013950a3a8b7a5a54"
   dependencies: