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

Why this change?

One Ruby class per file improves readability

lib/theme_settings_manager/bool.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class ThemeSettingsManager::Bool < ThemeSettingsManager
+  def self.cast(value)
+    [true, "true"].include?(value)
+  end
+
+  def value
+    self.class.cast(super)
+  end
+
+  def value=(new_value)
+    new_value = (self.class.cast(new_value)).to_s
+    super(new_value)
+  end
+end