[proposal] add is<JsonArray/Object/VariantConst>()

Created by: jjsuwa-sys3175

currently, to get a read-only JSON aggregate from its parent variant/document is such as:

JsonVariant var=doc[0];
if (var.is<JsonArray>()) {
  auto ro_arr=var.as<JsonArrayConst>();
  ...
}

in terms of the functional orthogonality, that can be:

JsonVariant var=doc[0];
if (var.is<JsonArrayConst>()) {
  auto ro_arr=var.as<JsonArrayConst>();
  ...
}