Add support for inverse buttons

Inverse buttons were added to govuk-frontend in PR #3556[0]

[0] alphagov/govuk-frontend#3556

lib/govuk_design_system_formbuilder/builder.rb

@@ -857,6 +857,7 @@ def govuk_check_box_divider(text = config.default_check_box_divider_text)
     # @param text [String,Proc] the button text. When a +Proc+ is provided its contents will be rendered within the button element
     # @param warning [Boolean] makes the button red ({https://design-system.service.gov.uk/components/button/#warning-buttons warning}) when true
     # @param secondary [Boolean] makes the button grey ({https://design-system.service.gov.uk/components/button/#secondary-buttons secondary}) when true
+    # @param inverse [Boolean] inverts the colours of the button. Note this isn't yet part of the design system.
     # @param prevent_double_click [Boolean] adds JavaScript to safeguard the
     #   form from being submitted more than once
     # @param validate [Boolean] adds the formnovalidate to the submit button when true, this disables all
@@ -881,8 +882,8 @@ def govuk_check_box_divider(text = config.default_check_box_divider_text)
     #   = f.govuk_submit "Proceed", prevent_double_click: true do
     #     = link_to 'Cancel', some_other_path, class: 'govuk-button__secondary'
     #
-    def govuk_submit(text = config.default_submit_button_text, warning: false, secondary: false, prevent_double_click: true, validate: config.default_submit_validate, disabled: false, **kwargs, &block)
-      Elements::Submit.new(self, text, warning: warning, secondary: secondary, prevent_double_click: prevent_double_click, validate: validate, disabled: disabled, **kwargs, &block).html
+    def govuk_submit(text = config.default_submit_button_text, warning: false, secondary: false, inverse: false, prevent_double_click: true, validate: config.default_submit_validate, disabled: false, **kwargs, &block)
+      Elements::Submit.new(self, text, warning: warning, secondary: secondary, inverse: inverse, prevent_double_click: prevent_double_click, validate: validate, disabled: disabled, **kwargs, &block).html
     end
 
     # Generates three inputs for the +day+, +month+ and +year+ components of a date

lib/govuk_design_system_formbuilder/elements/submit.rb

@@ -5,7 +5,7 @@ class Submit < Base
       include Traits::HTMLClasses
       include Traits::HTMLAttributes
 
-      def initialize(builder, text, warning:, secondary:, prevent_double_click:, validate:, disabled:, **kwargs, &block)
+      def initialize(builder, text, warning:, secondary:, inverse:, prevent_double_click:, validate:, disabled:, **kwargs, &block)
         super(builder, nil, nil)
 
         fail ArgumentError, 'buttons can be warning or secondary' if warning && secondary
@@ -14,6 +14,7 @@ def initialize(builder, text, warning:, secondary:, prevent_double_click:, valid
         @prevent_double_click = prevent_double_click
         @warning              = warning
         @secondary            = secondary
+        @inverse              = inverse
         @validate             = validate
         @disabled             = disabled
         @html_attributes      = kwargs
@@ -68,6 +69,7 @@ def classes
           "button--warning" => @warning,
           "button--secondary" => @secondary,
           "button--disabled" => @disabled,
+          "button--inverse" => @inverse,
         ).prefix(brand)
       end
     end