Improve detection of instance variables in extended modules

[HertzDevil]

Resolves #9568. This improves upon the current situation in two ways:

• The compiler currently doesn't detect instance variables that are declared with no types, i.e. whose types are deduced from their assignments. This PR adds support for these variables.
• The extending type is now shown in the error messages. This works even for chains of included types:

  module Foo
    @x : Int32? # Error: can't declare instance variables in Foo because Baz extends it
  end
  
  module Bar
    include Foo
  end
  
  module Baz
    extend Bar
  end

  The old message would have been can't declare instance variables in Baz:Module.

[beta-ziliani]

Thanks @HertzDevil !