Added Rails helpers

Necessary to extract from the `Filters` module since the current implementation uses a nested class. There's a couple of reasons why this is important:

* Use of a class is unnecessary and wasteful since there is no state. This is also known as the link:https://alchemists.io/articles/ruby_antipatterns#_nonclasses[Nonclass] antipattern.
* Due to the class having no state, the functionalitity can be fully encapsulated within a module as purely functional methods.
* Modules are the most lightweight and performant objects in Ruby.

We'll soon be able to replace the nested `Filters` class with this implementation.

Milestone: minor

lib/trmnl/liquid/rails_helpers.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require "action_view"
+
+module TRMNL
+  module Liquid
+    module RailsHelpers
+      extend ActionView::Helpers::TextHelper
+      extend ActionView::Helpers::NumberHelper
+    end
+  end
+end