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

Why this change?

One Ruby class per file improves readability

lib/theme_settings_manager/objects.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class ThemeSettingsManager::Objects < ThemeSettingsManager
+  def value
+    has_record? ? db_record.json_value : default.map!(&:deep_stringify_keys)
+  end
+
+  def value=(objects)
+    # TODO: Validate the objects against the schema
+
+    record = has_record? ? db_record : create_record!
+    record.json_value = objects
+    record.save!
+    theme.reload
+    record.json_value
+  end
+end