Fix ReScript deprecation warning: change es6 to esmodule

ReScript now recommends using 'esmodule' instead of the deprecated 'es6'
module format in package-specs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

bsconfig.json

@@ -8,7 +8,7 @@
   ],
   "package-specs": [
     {
-      "module": "es6",
+      "module": "esmodule",
       "in-source": true
     }
   ],