SHACL and SHACLC mime types

We're specifying how the Ontotext Platform should interop with SHACL, and
that includes returning SHACL and SHACLC.
(I believe that SHACLC is very promising because it's much more pleasant to
work with than SHACL, and closes one of the competitive gaps against SHEX.)

https://www.w3.org/TR/shacl and
https://w3c.github.io/shacl/shacl-compact-syntax/ don't specify file ext
and MIME types.
Here are some proposals:
- turtle: text/shacl+turtle with file ext: .shacl
- compact: text/shaclc with file ext .shaclc
- jsonld: application/shacl+ld+json (is that even valid?)
- rdf: application/shacl+rdf+xml (is that even valid?)

Holger, could you send a MIME registration request to IANA?