Mark package as being side-effect-free

For tree-shaking purposes with Webpack
(https://webpack.js.org/guides/tree-shaking/,
https://sgom.es/posts/2020-06-15-everything-you-never-wanted-to-know-about-side-effects/)

package.json

@@ -43,6 +43,7 @@
   "publishConfig": {
     "access": "public"
   },
+  "sideEffects": false,
   "scripts": {
     "build": "pnpm clean && tsc --project tsconfig.build.json && tsc --project tsconfig.build-esm.json",
     "clean": "rimraf dist coverage",