Support main object

[ksss]

RBS & Steep does not handle the main object well.

# sample.rb
using Foo

sample.rb:1:0: [error] Type `::Object` does not have method `using`
│ Diagnostic ID: Ruby::NoMethod
│
└ using Foo
  ~~~~~

Problem

Steep (1.10.0) treats main object as Object.

Proposal

We propose to introduce RBS::Unnamed::TopLevelSelfClass.
This is modeled on ENV and ARGF.

class RBS::Unnamed::TopLevelSelfClass
  def using: (Module arg0) -> self
end

Steep

Use RBS::Unnamed::TopLevelSelfClass as the main object in Steep.

Reference

#2357