Introduce field_name view helper

[seanpdoyle]

The field_name helper and corresponding FormBuilder#field_name
method provide an Action View-compliant way of overriding a form field
element's [name] attribute (similar to field_id and
FormBuilder#field_id introduced in #40127[][]).

text_field_tag :post, :title, name: field_name(:post, :title, :subtitle)
  # => <input type="text" name="post[title][subtitle]">

text_field_tag :post, :tag, name: field_name(:post, :tag, multiple: true)
  # => <input type="text" name="post[tag][]">

form_for @post do |f|
  f.field_tag :tag, name: f.field_name(:tag, multiple: true)
  # => <input type="text" name="post[tag][]">
end

[p8]

Nice! I think this is really useful to extract as a helper.