DEV: Refactor subclasses in ThemeSettingsManager to individual files (#25605)

Why this change?

One Ruby class per file improves readability

lib/theme_settings_manager/enum.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class ThemeSettingsManager::Enum < ThemeSettingsManager
+  def value
+    val = super
+    match = choices.find { |choice| choice == val || choice.to_s == val }
+    match || val
+  end
+
+  def is_valid_value?(new_value)
+    choices.include?(new_value) || choices.map(&:to_s).include?(new_value)
+  end
+
+  def choices
+    @opts[:choices]
+  end
+end