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/containsKey.cpp

@@ -25,15 +25,6 @@ TEST_CASE("JsonVariant::containsKey()") {
     REQUIRE(var.containsKey(std::string("hello")) == true);
     REQUIRE(var.containsKey(std::string("world")) == false);
   }
-
-  SECTION("linked object") {
-    StaticJsonDocument<128> doc2;
-    doc2["hello"] = "world";
-    var.link(doc2);
-
-    CHECK(var.containsKey("hello") == true);
-    CHECK(var.containsKey("world") == false);
-  }
 }
 
 TEST_CASE("JsonVariantConst::containsKey()") {