Recommend predicate for a literal media type (Content-Type)

This was mentioned during 2023-05-31CG meeting

4.2.1 Contained Resource Metadata states:

rdf:type
A class whose URI is the expansion of the URI Template [RFC6570] http://www.w3.org/ns/iana/media-types/{+iana-media-type}#Resource, where iana-media-type corresponds to a value from the IANA Media Types [IANA-MEDIA-TYPES].

Advertising the mime type with rdf:type has a couple of issues

• rdf:type property can have any number of values, mime-type should have none for RDFSources and exactly one for Non-RDFSources
• providing it as IRI requires the use of the IRI Template to compare it with the literal value of the registered media type

In sai-js I just picked an arbitrary http://bblfish.net/work/atom-owl/2006-06-06/#type.
In CSS I see usage of http://www.w3.org/ns/ma-ont#format, which seems a better choice

https://www.w3.org/TR/mediaont-10/#core-property-lists

format
The MIME type of the resource (e.g., wrapper or bucket media types, container types), ideally including as much information as possible about the resource such as media type parameters, for example, using the "codecs" parameter [RFC 4281].

For now, I'm going to align what sai-js uses with CSS (http://www.w3.org/ns/ma-ont#format). I think Solid Protocol should recommend this (or equivalent) predicate and use it in all relevant shapes.

Read section 4.2.1, the linked 2023-05-31 meeting notes, and the Media Ontology format property discussion. Compare the current Solid Protocol wording with sai-js and the relevant shapes; done means the predicate choice and its use across all relevant shapes are agreed and reflected in the specification.