Ensure id and type aliases are declared in scoped contexts.

Co-authored-by: Dave Longley <dlongley@digitalbazaar.com>

Author: msporny

contexts/credentials/v2

@@ -10,6 +10,9 @@
       "@context": {
         "@protected": true,
 
+        "id": "@id",
+        "type": "@type",
+
         "credentialSchema": {
           "@id": "https://www.w3.org/2018/credentials#credentialSchema",
           "@type": "@id"
@@ -74,6 +77,8 @@
       "@context": {
         "@protected": true,
 
+        "id": "@id",
+        "type": "@type",
         "holder": {
           "@id": "https://www.w3.org/2018/credentials#holder",
           "@type": "@id"