Mime type for validator schema

[hsolbrig]

The USVString part of the '''schema''' parameter for the validator api:
'''
Promise validate((USVString or RDFGraph) schema,
'''
could be a ShExC, ShExJ, ShExR, ... string. A mime type would be quite useful.

[gkellogg]

WebIDL is somewhat restricted in the datatypes available. URL parameters are commonly DOMString or USVString in other W3C Specs. Mime Types come in when interpreting the dereferenced result.