Extract shared T::Enum cop logic into a mixin

Introduce a mixin containing logic that will be shared between all cops that operate on `T::Enum`s.

lib/rubocop/cop/sorbet/mixin/t_enum.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Sorbet
+      # Mixing for writing cops that deal with `T::Enum`s
+      module TEnum
+        extend RuboCop::NodePattern::Macros
+        def initialize(*)
+          @scopes = []
+          super
+        end
+
+        # @!method t_enum?(node)
+        def_node_matcher :t_enum?, <<~PATTERN
+          (class (const...) (const (const nil? :T) :Enum) ...)
+        PATTERN
+
+        def on_class(node)
+          @scopes.push(node)
+        end
+
+        def after_class(node)
+          @scopes.pop
+        end
+
+        private
+
+        def in_t_enum_class?
+          t_enum?(@scopes&.last)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/sorbet_cops.rb

@@ -1,7 +1,8 @@
 # frozen_string_literal: true
 
-require_relative "sorbet/mixin/target_sorbet_version.rb"
-require_relative "sorbet/mixin/signature_help.rb"
+require_relative "sorbet/mixin/target_sorbet_version"
+require_relative "sorbet/mixin/t_enum"
+require_relative "sorbet/mixin/signature_help"
 
 require_relative "sorbet/binding_constant_without_type_alias"
 require_relative "sorbet/constants_from_strings"