fix: 🐛 mjs causing an elusive bug in webpack module resolution

Changed i18n.mjs to i18n.esm.js to prevent webpack to only accept .mjs
imports inside the library. Webpack was importing the cjs version of
Intl-Formatmessage and messing things up.

Issues: #36

package.json

@@ -1,8 +1,8 @@
 {
   "name": "svelte-i18n",
   "version": "2.2.1",
-  "main": "dist/i18n.js",
-  "module": "dist/i18n.mjs",
+  "main": "dist/i18n.cjs.js",
+  "module": "dist/i18n.esm.js",
   "bin": {
     "svelte-i18n": "dist/cli.js"
   },