feat: mark as side effect free to enable efficient tree-shaking (#423)

Author: ktsn

package.json

@@ -6,6 +6,7 @@
   "module": "dist/vue-class-component.esm.js",
   "unpkg": "dist/vue-class-component.js",
   "typings": "lib/index.d.ts",
+  "sideEffects": false,
   "files": [
     "dist",
     "lib",