Add support for preprocessed files

[fantazio]

Add tests using ppx and add support for preprocessed files.

This contributes to https://github.com/LexiFi/dead_code_analyzer/issues/40 for OCaml 5.2. It applies the suggested solution. It works well (based on the tests results) for OCaml 5.2 but is not sufficient for 5.3. This is still an improvement for 5.3. I'd prefer to merge this PR before 1.0.1 to include the support in that release. I don't plan on having another intermediate release before OCaml 5.3 compatibility but I am opened to discuss this.