Easier Initialization of glz::json

[mwalcott3]

Modeled after the interface used in nlohmann https://github.com/nlohmann/json#json-as-first-class-data-type
Made in response to the discussion item #130 posted by @klemenpeganc

Allows for this:

glz::json_t json = {
   {"pi", 3.141},
   {"happy", true},
   {"name", "Niels"},
   {"nothing", nullptr},
   {"answer", {{"everything", 42.0}}},
   {"list", {1.0, 0.0, 2.0}},
   {"object", {
      {"currency", "USD"},
      {"value", 42.99}
   }}
};

Or this:

glz::json_t json;
json["pi"] = 3.14;
json["happy"] = true;
json["name"] = "Niels";
json["nothing"] = nullptr;
json["answer"]["everything"] = 42.0;
json["list"] = { 1.0, 0.0, 2.0 };
json["object"] = { {"currency", "USD"}, {"value", 42.99} };