Add @top_level to access the top-level scope in macros

[beta-ziliani]

Fixes #8914 by creating a new instance method @top_level (as suggested by @jhass). This allows to query the top level module, as the following example shows:

FOO = 1
class Foo
   def bar
       {{@top_level.has_constant?("FOO")}}
    end
end

[asterite]

Looks great, thank you!

[straight-shoota]

Gracias, @beta-ziliani