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/or.cpp

@@ -156,12 +156,4 @@ TEST_CASE("JsonVariant::operator|()") {
     int result = variant | 42;
     REQUIRE(result == 42);
   }
-
-  SECTION("linked int | int") {
-    StaticJsonDocument<128> doc2;
-    doc2.set(42);
-    variant.link(doc2);
-    int result = variant | 666;
-    CHECK(result == 42);
-  }
 }