OCaml 5.3 compatibility

[fantazio]

The current compatible OCaml version is 5.2. The latest OCaml release is 5.4.0.

There is interest in having an OCaml 5.3 compatible version (https://github.com/LexiFi/dead_code_analyzer/pull/17#issuecomment-3199461320, and https://github.com/LexiFi/dead_code_analyzer/pull/31#issuecomment-3603188491). This would also provide a strong intermediate step before catching up with the latest release.

Creating this issue to help keep track of sub issues related to the 5.3 compatibility.