[meta] add sideEffects flag

Author: ljharb

Diff for: package.json

@@ -8,6 +8,7 @@
 	"description": "Is this value a JS Date object? This module works cross-realm/iframe, and despite ES6 @@toStringTag.",
 	"license": "MIT",
 	"main": "index.js",
+	"sideEffects": false,
 	"scripts": {
 		"prepack": "npmignore --auto --commentLines=autogenerated",
 		"prepublishOnly": "safe-publish-latest",