Update w3c schema to 2020 format

The `thing-context` definition used a 2019 draft of how to specify a
tuple (where `items` set the mandatory items and `additionalItems`
set the rest).  These have been renamed to `prefixItems` and `items`
respectively, and I have applied this change.  Schema now validates
again :)

tests/w3c_td_schema.json

@@ -66,10 +66,10 @@
         "thing-context": {
             "oneOf": [{
                     "type": "array",
-                    "items": [{
+                    "prefixItems": [{
                         "$ref": "#/definitions/thing-context-w3c-uri"
                     }],
-                    "additionalItems": {
+                    "items": {
                         "anyOf": [{
                                 "$ref": "#/definitions/anyUri"
                             },