Implement yard directive visibility handling

[ghost]

Implement handling of the yard @!visibility directive.

Should address #566.

Signed-off-by: liberatys nick.anthony.flueckiger@hey.com

[castwide]

This is a great start, but there are still a couple of use cases that don't match YARD's specification.

1. A @!visibility directive that is attached to a method should not modify other methods. Example spec:

   it "processes private visibility directives attached to methods" do
     map = Solargraph::SourceMap.load_string(%(
       class Example
         # @!visibility private
         def method1; end
   
         def method2; end
       end
     ))
     method1 = map.first_pin('Example#method1')
     expect(method1.visibility).to be(:private)
     method2 = map.first_pin('Example#method2')
     expect(method2.visibility).to be(:public)
   end

2. A @!visibility directive that is not attached to a method should modify other methods and can be overridden. Example spec:

   it "processes class-wide private visibility directives" do
     map = Solargraph::SourceMap.load_string(%(
       class Example
         # @!visibility private
   
         def method1; end
   
         def method2; end
   
         # @!visibility public
         def method3; end
       end
     ))
     method1 = map.first_pin('Example#method1')
     expect(method1.visibility).to be(:private)
     method2 = map.first_pin('Example#method2')
     expect(method2.visibility).to be(:private)
     method3 = map.first_pin('Example#method3')
     expect(method3.visibility).to be(:public)
   end

[castwide]

I'll go ahead and merge this and make the changes to pass the two additional specs. It'll be released in 0.47.0.