Add english transaction for project structure doc (separateexpress)

Description

I noticed that multiple language versions are available for separateexpress, but the English version is missing from the documentation.

Why this matters

Since this repository aims to provide Node.js best practices to a global audience, having an English version of the project structure documentation would improve accessibility and usability for a wider community.

Suggested solution

• Add an English translation of separateexpress.chinese.md (as an example; other language variants exist as well)
• Follow the existing naming convention used for English files (e.g., separateexpress.md)

Contribution

I’d be happy to help with the translation if this is something you’d like to include.