Make Style/TrailingBodyOnClass aware of singleton class

This PR makes `Style/TrailingBodyOnClass` aware of singleton class.
So, singleton class can be handled in the same way as class.

changelog/new_make_style_trailing_body_on_class_aware_of_singleton_class.md

@@ -0,0 +1 @@
+* [#11775](https://github.com/rubocop/rubocop/pull/11775): Make `Style/TrailingBodyOnClass` aware of singleton class. ([@koic][])

lib/rubocop/cop/style/trailing_body_on_class.rb

@@ -34,6 +34,7 @@ def on_class(node)
             )
           end
         end
+        alias on_sclass on_class
       end
     end
   end

spec/rubocop/cop/style/trailing_body_on_class_spec.rb

@@ -35,6 +35,26 @@ def bar; end
     RUBY
   end
 
+  it 'registers an offense when body trails after singleton class definition' do
+    expect_offense(<<~RUBY)
+      class << self; body
+                     ^^^^ Place the first line of class body on its own line.
+      end
+      class << self; def bar; end
+                     ^^^^^^^^^^^^ Place the first line of class body on its own line.
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      class << self#{trailing_whitespace}
+        body
+      end
+      class << self#{trailing_whitespace}
+        def bar; end
+      end
+    RUBY
+  end
+
   it 'registers offense with multi-line class' do
     expect_offense(<<~RUBY)
       class Foo; body