More compact @prefix

Currently, @prefix has boolean as range, and can be used as follows:

{
  "@context": {
    "@version": 1.1,
    "compact-iris": { "@id": "http://example.com/compact-iris-", "@prefix": true }
  },
}

In most cases, @prefix will probably be used together with @id.
For this reason, it might be beneficial to also allow the following as equivalent representation:

{
  "@context": {
    "@version": 1.1,
    "compact-iris": { "@prefix": "http://example.com/compact-iris-" }
  },
}

As far as I can see, this should not cause conflicts or major performance issues when processing.

No file or test is named in the issue. Begin by reading the JSON-LD 1.1 context-processing specification around @prefix and @id; done means the string form is defined as equivalent to the existing representation without introducing conflicts.