Fix weird error when adding library to real app

When attempting to test integration between the gem and a real Rails
app, I received a weird error coming from the
`raise_if_slot_ends_with_question_mark` method inside the
`view_component` gem. This didn't happen when I last tested the
integration, which is very odd indeed.

Relevant PR: ViewComponent/view_component#1905

lib/citizens_advice_form_builder/form_builder.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "action_view"
+require "active_support/core_ext/string/starts_ends_with"
 require "citizens_advice_components"
 
 require CitizensAdviceComponents::Engine.root.join("app", "lib", "citizens_advice_components", "fetch_or_fallback_helper")