feat(context): use "@type" instead of "rdf:type"

[ethieblin]

Similarly to #1466

The input data of these examples was generated by transforming a TD into RDF and the RDF into json-ld with rdflib
(same result when uploading/downloading the RDF triples from a SPARQL endpoint)

"type" does not round trip on the TD properties type

playground with original context

"properties": {
    "airpressure": {
      "@type": "number",
      }
 }

playground with new context

"properties": {
    "airpressure": {
      "type": "number",
      }
 }

[ethieblin]

@wiresio fyi

[sebastiankb]

same situation: see #1466 (comment)

[sebastiankb]

same situation: see #1466 (comment)

[sebastiankb]

from today's TD call, decided to merge

[w3cbot]

ashimura marked as non substantive for IPR from ash-nazg.