feat: mark angular packages as side-effect free (#22785)

This flag is picked up by webpack v4 and used for more agressive optimizations.

Our code is already side-effect free, because that's what we needed for build-optimizer to work.

PR Close #22785

packages/animations/browser/package.json

@@ -3,5 +3,6 @@
   "typings": "./browser.d.ts",
   "main": "../bundles/animations-browser.umd.js",
   "module": "../esm5/browser.js",
-  "es2015": "../esm2015/browser.js"
+  "es2015": "../esm2015/browser.js",
+  "sideEffects": false
 }

packages/animations/package.json

@@ -20,5 +20,6 @@
   },
   "ng-update": {
     "packageGroup": "NG_UPDATE_PACKAGE_GROUP"
-  }
+  },
+  "sideEffects": false
 }