produced JSON files should have the `.json` extension as suggested by `RFC 8259`

[lzaoral]

We should probably change the extension from .js to .json as recommended by the RFC defining JSON: https://datatracker.ietf.org/doc/html/rfc8259#section-11

Originally reported in openscanhub/openscanhub#186 (comment).

[kdudka]

One way to do this in a backward-compatible way would be to provide symlinks with the .js suffix like we did in #124.