docs: update migration guide with pure ESM package info (#140)

Author: chenjiahan

docs/migrate-v1-to-v2.md

@@ -14,6 +14,10 @@ The minimum supported Node.js version is now `^20.19.0 || >=22.12.0`.
 
 `@rspack/dev-server` v2 is designed to work with Rspack v2. Rspack v1 is no longer supported in v2.
 
+## Pure ESM package
+
+`@rspack/dev-server` is now published as **pure ESM** package.
+
 ### Upgraded `http-proxy-middleware` to v3
 
 `http-proxy-middleware` has been updated to v3, which has some breaking changes: