fix: esm for new flat config

Author: amritk

package.json

@@ -21,7 +21,7 @@
   ],
   "exports": {
     ".": {
-      "import": "./lib/index.esm.js",
+      "import": "./lib/index.mjs",
       "require": "./lib/index.cjs.js",
       "default": "./lib/index.cjs.js"
     },

rollup.config.js

@@ -30,7 +30,7 @@ export default [
     external,
     output: [
       { dir: 'lib', entryFileNames: '[name].cjs.js', format: 'cjs' },
-      { dir: 'lib', entryFileNames: '[name].esm.js', format: 'es' },
+      { dir: 'lib', entryFileNames: '[name].mjs', format: 'es' },
     ],
     plugins: [commonjs(), resolve(), typescript(), json()],
   },