Use `application/vnd.` prefix instead `chemical/x-` to register it at IANA

[tpoisseau]

To register a media type: https://www.iana.org/form/media-types

It should probably start with application/vnd.: https://datatracker.ietf.org/doc/html/rfc6838#section-3.2

Only registered types are accepted by browsers (open in a web app for example).

Originally posted by @targos in #3708