Change link() to shallowCopy() (issue #1343)

Instead of storing a pointer, the function copies the `VariantData`.

Benefits:
* smaller code
* no impact on programs that don't use this feature

Drawbacks:
* changes to the original variant are not always reflected on the copy
* modifying the original from the shallow copy leads to UB

extras/tests/JsonVariant/add.cpp

@@ -43,14 +43,4 @@ TEST_CASE("JsonVariant::add()") {
 
     REQUIRE(var.as<std::string>() == "{\"val\":123}");
   }
-
-  SECTION("add to linked array") {
-    StaticJsonDocument<1024> doc2;
-    doc2.add(42);
-    var.link(doc2);
-
-    var.add(666);  // no-op
-
-    CHECK(var.as<std::string>() == "[42]");
-  }
 }