Add support for 'package' in expressions. (#3445)

This creates a namespace for `package` scope.

It looks like names of class_decls in namespaces lead to an unexpected
instref. This is already true, as best as I can tell. I'm not sure if
there's a preferred approach to address that, so I've left a TODO for
now.

toolchain/parse/handle_expr.cpp

@@ -147,6 +147,11 @@ auto HandleExprInPostfix(Context& context) -> void {
       context.PushState(State::ArrayExpr);
       break;
     }
+    case Lex::TokenKind::Package: {
+      context.AddLeafNode(NodeKind::PackageExpr, context.Consume());
+      context.PushState(state);
+      break;
+    }
     case Lex::TokenKind::SelfValueIdentifier: {
       context.AddLeafNode(NodeKind::SelfValueNameExpr, context.Consume());
       context.PushState(state);