Add JsonVariant::link() (resolves #1343)

extras/tests/JsonDocument/ElementProxy.cpp

@@ -245,3 +245,11 @@ TEST_CASE("ElementProxy cast to JsonVariant") {
 
   CHECK(doc.as<std::string>() == "[\"toto\"]");
 }
+
+TEST_CASE("ElementProxy::link()") {
+  StaticJsonDocument<1024> doc1, doc2;
+  doc1[0].link(doc2);
+  doc2["hello"] = "world";
+
+  CHECK(doc1.as<std::string>() == "[{\"hello\":\"world\"}]");
+}