Fix WebIDL (which currently means patching ReSpec) and introduce two typedefs

The two typedefs are

    typedef (object or object[] or DOMString) JsonLdInput;
    typedef (object or DOMString) JsonLdContext;

and make the interface definition much easier to read.

I've filed two PR for the patches to ReSpec (https://github.com/darobin/respec/pull/190, https://github.com/darobin/respec/pull/191).