Add support for methods defined with `define_singleton_method`

[uvlad7]

So, I don't have any Rust struct to wrap, I just make a Ruby binding for a third-party crate. So, the methods are defined with define_singleton_method, and yard-rustdoc doesn't seem to work with them.

I ended up doing this, it's fine for this specific gem, but I actually want to use a more clean solution - like rustdoc - for docs, because I want this project to become a template of wrapping a Rust library into a Ruby/Jruby gem, a Python package, etc.

Btw documenting modules/classes created with define_module/define_class would be nice, too.

[ianks]

Hey @uvlad7 thanks for the report! There's definitely some gaps with the plugin that we would love some help with. Thanks for reporting this. Turning this into an issue for someone to pick up