Rename Type -> type, per #2360. (#2507)

Also make minor updates to the skeletal design in
docs/design/name_lookup.md following #2113, as there are no longer any prelude names that are made available to unqualified name lookup by default.

Add `type` to the keyword list in
docs/design/lexical_conventions/words.md, following #2360.

explorer/ast/expression.cpp

@@ -329,7 +329,7 @@ void Expression::PrintID(llvm::raw_ostream& out) const {
       out << "String";
       break;
     case ExpressionKind::TypeTypeLiteral:
-      out << "Type";
+      out << "type";
       break;
     case ExpressionKind::ContinuationTypeLiteral:
       out << "Continuation";