Add more advanced `initializer_list` support

[jviotti]

Like https://github.com/nlohmann/json does:

json j2 = {
  {"pi", 3.141},
  {"happy", true},
  {"name", "Niels"},
  {"nothing", nullptr},
  {"answer", {
    {"everything", 42}
  }},
  {"list", {1, 0, 2}},
  {"object", {
    {"currency", "USD"},
    {"value", 42.99}
  }}
};

[tony-go]

Useful link: https://github.com/nlohmann/json?tab=readme-ov-file#creating-json-objects-from-json-literals

[jviotti]

Exactly. The point being that otherwise we often have to create stringified JSON documents and pass them through sourcemeta::jsontoolkit::parse() (which is relatively slow), while with these kind of literals, we directly allocate the document and avoid parsing (much faster).