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

extras/tests/JsonVariant/clear.cpp

@@ -23,4 +23,15 @@ TEST_CASE("JsonVariant::clear()") {
 
     REQUIRE(var.isNull() == true);
   }
+
+  SECTION("doesn't alter linked object") {
+    StaticJsonDocument<128> doc2;
+    doc2["hello"] = "world";
+    var.link(doc2);
+
+    var.clear();
+
+    CHECK(var.isNull() == true);
+    CHECK(doc2.as<std::string>() == "{\"hello\":\"world\"}");
+  }
 }