feat(size): add sideEffects false to package.json (#2861)

Why is this done? see <https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free>

Long story short, modules can do crazy things, but if we don't do any polyfilling, we can opt into better behaviour

fixes #2859 as good as we can for now

Author: Haroenv

package.json

@@ -14,6 +14,7 @@
   "main": "dist-es5-module/index.js",
   "module": "./es/index.js",
   "jsdelivr": "dist/instantsearch.min.js",
+  "sideEffects": false,
   "scripts": {
     "build": "./scripts/build.sh",
     "dev": "./scripts/dev.sh",