refactor(css): Remove unnecessary and structure code, add comments

[hyf0]

Related issue (if exists)

#3449

Summary

🤖 Generated by Copilot at 86f789c

Improved the CSS plugin for rspack and updated the test case for data imports. The plugin can now generate the CSS source for a chunk of modules, and the test case uses a snapshot to verify the output.

Walkthrough

🤖 Generated by Copilot at 86f789c

• Refactor CSS plugin logic and update data imports test case
• Add render_chunk_to_source method to CssPlugin to generate CSS source for a chunk (link)
• Use render_chunk_to_source method and simplify render_manifest hook logic (link, link)
• Update imports to include Chunk and ModuleIdentifier types and remove unused type (link)
• Use snapshot for expected CSS content in data-imports test case (packages/rspack/tests/cases/schemes/data-imports/index.js, link)