fix: update module and exports paths to use .mjs extension for ESM compatibility

- sideEffects options to false in package.json

Author: kalwalt

README.md

@@ -110,7 +110,7 @@ npm run build:vite
 
 **Outputs** (written to dist/)
 
-- `dist/arjs-core.es.js` (ESM — used by `exports.import` / `module`)
+- `dist/arjs-core.mjs` (ESM — used by `exports.import` / `module`)
 - `dist/arjs-core.js` (CommonJS — used by `exports.require` / `main`)
 
 **Notes**

package.json

@@ -3,14 +3,15 @@
   "version": "0.2.0",
   "description": "core library of AR.js",
   "main": "dist/arjs-core.js",
-  "module": "dist/arjs-core.es.js",
+  "module": "dist/arjs-core.mjs",
   "exports": {
     ".": {
-      "import": "./dist/arjs-core.es.js",
+      "import": "./dist/arjs-core.mjs",
       "require": "./dist/arjs-core.js",
-      "default": "./dist/arjs-core.es.js"
+      "default": "./dist/arjs-core.mjs"
     }
   },
+  "sideEffects": false,
   "scripts": {
     "format-check": "prettier --check .",
     "format": "prettier --write .",