fix: set sideEffects to false (#286)

Allows bundlers to tree-shake out the temporal polyfill when imported but not used.

package.json

@@ -22,6 +22,7 @@
       "./dist/index.cjs"
     ]
   },
+  "sideEffects": false,
   "types": "./index.d.ts",
   "scripts": {
     "test": "npm run build && node --no-warnings --experimental-modules --experimental-specifier-resolution=node --icu-data-dir node_modules/full-icu --loader ./test/resolve.source.mjs ./test/all.mjs",