FEATURE: Allow themes to specify modifiers in their about.json file (#9097)

There are three modifiers:
- serialize_topic_excerpts (boolean)
- csp_extensions (array of strings)
- svg_icons (array of strings)

When multiple themes are active, the values will be combined. The combination method varies based on the setting. CSP/SVG arrays will be combined. serialize_topic_excerpts will use `Enumerable#any`.

lib/theme_modifier_helper.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+class ThemeModifierHelper
+  def initialize(request: nil, theme_ids: nil)
+    @theme_ids = theme_ids || request&.env&.[](:resolved_theme_ids)
+  end
+
+  ThemeModifierSet::MODIFIERS.keys.each do |modifier|
+    define_method(modifier) do
+      Theme.lookup_modifier(@theme_ids, modifier)
+    end
+  end
+end