Make it possible to chain source maps correctly on webpack and rspack #561

Describe the feature

Webpack and rspack do not automatically chain the source maps returned by loaders with source maps generated earlier in the loader chain; each loader needs to combine the source map it generates with the source map generated by the previous loader. Unplugin does not expose the input source map to the transform handler, so it is not possible for Unplugin-based plugins to handle this correctly today.

There are various approaches we could take to solve this problem, but the simplest would be to expose to input source map to the transform handler.

Additional information

• Would you be willing to help implement this feature?