add changelog

Author: cometkim

CHANGELOG.md

@@ -16,6 +16,10 @@
 
 - Fix `Error.fromException`. https://github.com/rescript-lang/rescript/pull/7364
 
+#### :house: Internal
+
+- Convert internal JavaScript codebase into ESM, ReScript package itself is now ESM (`"type": "module"`). https://github.com/rescript-lang/rescript/pull/6899
+
 # 12.0.0-alpha.10
 
 #### :rocket: New Feature