Removed JsonArray::is<T>(i) and JsonArray::set(i,v)

Removed `JsonObject::is<T>(k)` and `JsonObject::set(k,v)`
Replaced `T JsonArray::get<T>(i)` with `JsonVariant JsonArray::get(i)`
Replaced `T JsonObject::get<T>(k)` with `JsonVariant JsonObject::get(k)`

test/JsonObject/subscript.cpp

@@ -86,7 +86,7 @@ TEST_CASE("JsonObject::operator[]") {
   SECTION("object subscript") {
     DynamicJsonDocument doc2;
     JsonObject obj2 = doc2.to<JsonObject>();
-    obj2.set("x", 42);
+    obj2["x"] = 42;
 
     obj["a"] = obj2["x"];