Mark formik as side-effect free (#3501)

This will allow Formik's code to be removed by tree-shaking bundlers like Webpack or esbuild.

Author: markspolakovs

.changeset/wise-games-battle.md

@@ -0,0 +1,5 @@
+---
+"formik": patch
+---
+
+Mark `formik` as side-effect free in `package.json`

packages/formik/package.json

@@ -37,6 +37,7 @@
   "peerDependencies": {
     "react": ">=16.8.0"
   },
+  "sideEffects": false,
   "scripts": {
     "test": "tsdx test --env=jsdom",
     "test:watch": "npm run test -- --watchAll",