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/nesting.cpp

@@ -28,12 +28,4 @@ TEST_CASE("JsonVariant::nesting()") {
     var.to<JsonArray>();
     REQUIRE(var.nesting() == 1);
   }
-
-  SECTION("returns depth of linked array") {
-    StaticJsonDocument<128> doc2;
-    doc2[0][0] = 42;
-    var.link(doc2);
-
-    CHECK(var.nesting() == 2);
-  }
 }