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/size.cpp

@@ -33,21 +33,4 @@ TEST_CASE("JsonVariant::size()") {
 
     CHECK(variant.size() == 2);
   }
-
-  SECTION("linked object") {
-    StaticJsonDocument<1024> doc2;
-    doc2["hello"] = "world";
-    variant.link(doc2);
-
-    CHECK(variant.size() == 1);
-  }
-
-  SECTION("linked array") {
-    StaticJsonDocument<1024> doc2;
-    doc2.add(1);
-    doc2.add(2);
-    variant.link(doc2);
-
-    CHECK(variant.size() == 2);
-  }
 }