Cop/StaticTranslationDefinition: Allow constant assignments for Structs

Prior this commit, Cop/StaticTranslationDefinition would flag the
following code:

    SomeClass = Struct.new do
      def text
        _('Some translated text')
      end
    end

This commit allow such style again so workarounds are not longer needed
like:

    Struct.new('SomeClass') do
      def text
        _('Some translated text')
      end
    end

rubocop/cop/static_translation_definition.rb

@@ -15,6 +15,10 @@ module RuboCop
         (send _ :lambda)
       PATTERN
 
+      def_node_matcher :struct_constant_assignment?, <<~PATTERN
+        (casgn _ _ `(const _ :Struct))
+      PATTERN
+
       def on_send(node)
         return unless translation_method?(node)
 
@@ -27,7 +31,7 @@ module RuboCop
           receiver, _ = *ancestor
           break if lambda_node?(receiver) # translations defined in lambda nodes should be allowed
 
-          if constant_assignment?(ancestor)
+          if constant_assignment?(ancestor) && !struct_constant_assignment?(ancestor)
             add_offense(node, location: :expression)
 
             break

spec/rubocop/cop/static_translation_definition_spec.rb

@@ -112,7 +112,7 @@ RSpec.describe RuboCop::Cop::StaticTranslationDefinition do
             }
           end
         CODE
-        <<~CODE
+        <<~CODE,
           class MyClass
             def hello
               {
@@ -121,6 +121,20 @@ RSpec.describe RuboCop::Cop::StaticTranslationDefinition do
             end
           end
         CODE
+        <<~CODE,
+          SomeClass = Struct.new do
+            def text
+              _('Some translated text')
+            end
+          end
+        CODE
+        <<~CODE
+          Struct.new('SomeClass') do
+            def text
+              _('Some translated text')
+            end
+          end
+        CODE
       ]
     end