Add rich_text_area

[kalashnikovisme]

What's changed basically?

Added Tramway support for Action Text rich text fields.

• Tramway::Form::Builder#rich_text_area
• Tramway::Form::Builder#rich_textarea
• tramway_field :rich_text_area
• Dark Tailwind/Trix editor and toolbar styling
• Form titles on edit pages now use the record title when available
• README docs were updated with the new helper

What's changed for tramway drivers?

Tramway forms can now render Action Text/Trix editors with Tramway form styling.

Before

class PostForm < Tramway::BaseForm
  fields :title, :text
end

<%= f.text_area :text %>

After

  class PostForm < Tramway::BaseForm
    fields :title, :rich_text_area
  end

  <%= f.rich_text_area :bio %>

rich_text_area keeps custom classes/data attributes and marks the editor so Tramway
JavaScript can style the Trix toolbar consistently with the dark form UI.

Docs updated in README:
https://github.com/Purple-Magic/tramway/blob/rich_text_area/README.md#tramway_form_for