Bindable CXX lambdas as methods in classes

[YakoYakoYokuYoku]

This PR allows API users to bind lambdas as member methods in classes for the bindings, much like Python language C++ bindings such as pybind11. This is a draft implementation of this godotengine/godot-proposals#14485 proposal.

Sample usage.

void Sample::_bind_methods() {
  ClassDB::bind_method(godot::D_METHOD("get_test"), [](const Sample &s) { return s.test; });
  ClassDB::bind_method(godot::D_METHOD("set_test"), [](Sample &s, const String &test) { s.test = test; });
}

See godotengine/godot#117541 for the engine implementation counterpart.

[dsnopek]

Personally, I don't think this is something we should merge into godot-cpp itself. One of godot-cpp's pillars is that's API compatible with Godot, and while we don't achieve that 100%, we shouldn't add new things that are incompatible.

This could be provided as a third-party header-only library for folks who want this convenience and don't need module compatibility.

[YakoYakoYokuYoku]

Could you wait for a sec so that I can confirm that this is doable outside the bindings? Just to put it to rest properly.

[YakoYakoYokuYoku]

I've confirmed that I could do this with an external library, I'm closing this.