Add support for extend self to handle_module_operation

By adding the instance methods as class methods through the
singleton mixin operations, we are able to get all instance
methods "copied" over into the class methods implicitly.

I don't think it's necessary to test whether this extend self
applies when the module is opened after being closed again,
or re-opened in another file.

Closes Shopify#2782

lib/ruby_indexer/lib/ruby_indexer/declaration_listener.rb

@@ -746,7 +746,8 @@ def handle_module_operation(node, operation)
       return unless arguments
 
       arguments.each do |node|
-        next unless node.is_a?(Prism::ConstantReadNode) || node.is_a?(Prism::ConstantPathNode)
+        next unless node.is_a?(Prism::ConstantReadNode) || node.is_a?(Prism::ConstantPathNode) ||
+          (node.is_a?(Prism::SelfNode) && operation == :extend)
 
         case operation
         when :include
@@ -755,7 +756,11 @@ def handle_module_operation(node, operation)
           owner.mixin_operations << Entry::Prepend.new(node.full_name)
         when :extend
           singleton = @index.existing_or_new_singleton_class(owner.name)
-          singleton.mixin_operations << Entry::Include.new(node.full_name)
+          singleton.mixin_operations << if node.is_a?(Prism::SelfNode)
+            Entry::Include.new(owner.name)
+          else
+            Entry::Include.new(node.full_name)
+          end
         end
       rescue Prism::ConstantPathNode::DynamicPartsInConstantPathError,
              Prism::ConstantPathNode::MissingNodesInConstantPathError

lib/ruby_indexer/test/index_test.rb

@@ -1672,6 +1672,26 @@ def test_linearizing_singleton_object
       )
     end
 
+    def test_extend_self
+      @index.index_single(IndexablePath.new(nil, "/fake/path/foo.rb"), <<~RUBY)
+        module Foo
+          def bar
+          end
+
+          extend self
+
+          def baz
+          end
+        end
+      RUBY
+
+      ["bar", "baz"].product(["Foo", "Foo::<Class:Foo>"]).each do |method, receiver|
+        entry = @index.resolve_method(method, receiver)&.first
+        refute_nil(entry)
+        assert_equal(method, T.must(entry).name)
+      end
+    end
+
     def test_linearizing_singleton_ancestors
       @index.index_single(IndexablePath.new(nil, "/fake/path/foo.rb"), <<~RUBY)
         module First