feat: mark the snabbdom package as side effect free

This is useful for bundlers. When dealing with reexports, this setting
allows e.g. webpack to optimize reexports. See the webpack documentation
for more information about this setting

package.json

@@ -24,6 +24,7 @@
   },
   "main": "build/snabbdom.cjs.js",
   "module": "build/index.js",
+  "sideEffects": false,
   "scripts": {
     "build": "tsc && npm run bundle:cjs",
     "bundle:cjs": "rollup build/index.js --format cjs --file build/snabbdom.cjs.js",