build: Use es6 target instead of esnext for ESM builds

Author: kamilogorek

tsconfig.esm.json

@@ -1,7 +1,7 @@
 {
   "extends": "./tsconfig.json",
   "compilerOptions": {
-    "target": "esnext",
-    "module": "esnext"
+    "target": "es6",
+    "module": "es6"
   }
 }