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/clear.cpp

@@ -23,15 +23,4 @@ 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\"}");
-  }
 }