Consider adding file reference for strings extracted from file headers

[swissspidy]

Feature Request

• [x] Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

Context:

• https://github.com/swissspidy/preferred-languages/issues/883
• https://meta.trac.wordpress.org/ticket/7376

Since there is no file reference for strings extracted from plugin/theme headers, it's easy to miss where they're from.

This was originally not added to keep feature parity with the old make-pot tool, but it seems reasonable to add.

Describe the solution you'd like

Add source file reference for things like plugin descriptions to avoid issues like the above.