feat: only split async chunks for umd format

[Timeless0911]

Summary

sync chunk of umd can not be loaded without mounting on scripts

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[Copilot]

Pull Request Overview

This PR modifies the UMD format configuration to only split asynchronous chunks, addressing a limitation where synchronous chunks cannot be loaded without being mounted on scripts.

• Updated splitChunks configuration to only split async chunks for UMD format
• Added corresponding test snapshots to reflect the configuration change

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/core/src/config.ts	Added splitChunks configuration with chunks: 'async' for UMD format
packages/core/tests/snapshots/config.test.ts.snap	Updated test snapshots to reflect the new splitChunks configuration

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}