mark the package as side-effects free (#189)

Help tree shaking tools to decide whether it's safe to drop this
package.

More info:
- https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free
- https://esbuild.github.io/api/#tree-shaking

package.json

@@ -2,6 +2,7 @@
   "name": "color",
   "version": "4.2.2",
   "description": "Color conversion and manipulation with CSS string support",
+  "sideEffects": false,
   "keywords": [
     "color",
     "colour",