Support error_items for specific input within date-input

As suggested in #3510.

This changes the date-input component so it supports errors on
individual input elements, e.g. just the year input element. This usage
is described in the docs for the date-input component [1]:

> If you’re highlighting just one field - either the day, month or year
> - only style the field that has an error. The error message must say
> which field has an error, like this:

However, highlighting just one field like this is not actually currently
supported by the date-input component!

The new behaviour (where individual fields are highlighted as having an
error) is specified by adding values for a `name` key to the
`error_items` with the values matching the values of the `name` key in
the `items`. If none of the `error_items` have a value for the `name`
key, then we fallback to the original behaviour, i.e. all fields are
highlighted as having an error. This should hopefully make this a fairly
backwardly-compatible change.

[1]: https://design-system.service.gov.uk/components/date-input/#error-messages

app/views/govuk_publishing_components/components/_date_input.html.erb

@@ -15,6 +15,8 @@
   error_items ||= []
   describedby ||= nil
   has_error ||= error_message || error_items.any?
+  error_item_names = error_items.group_by { |ei| ei[:name] }.keys.compact_blank
+  has_no_named_errors = error_item_names.none?
 
   css_classes = %w(gem-c-date-input govuk-date-input)
   form_group_css_classes = %w(govuk-form-group)
@@ -59,7 +61,7 @@
               text: item[:label] || item[:name].capitalize
             },
             grouped: true,
-            has_error: has_error,
+            has_error: error_item_names.include?(item[:name]) || (has_error && has_no_named_errors),
             name: name ? (name + "[" + item[:name] + "]") : item[:name],
             value: item[:value],
             width: item[:width],

app/views/govuk_publishing_components/components/docs/date_input.yml

@@ -69,3 +69,10 @@ examples:
           name: dob-blwyddyn
           width: 4
           value: 1980
+  with_named_error_items:
+    data:
+      legend_text: "When was your passport issued?"
+      hint: "For example, 27 3 2007"
+      error_items:
+        - name: "year"
+          text: "The date your passport was issued must include a year"

spec/components/date_input_spec.rb

@@ -78,6 +78,22 @@ def component_name
     assert_select ".govuk-form-group--error .govuk-fieldset[role=group] .govuk-date-input__item .govuk-input--error", 3
   end
 
+  it "renders with named error items" do
+    render_component(
+      legend_text: "What is your date of birth?",
+      error_items: [
+        { name: "day", text: "day-error" },
+        { name: "year", text: "year-error" },
+      ],
+    )
+
+    assert_select(
+      ".govuk-form-group--error .govuk-fieldset[role=group] .govuk-error-message",
+      html: "<span class=\"govuk-visually-hidden\">Error:</span> day-error<br>year-error",
+    )
+    assert_select ".govuk-form-group--error .govuk-fieldset[role=group] .govuk-date-input__item .govuk-input--error", 2
+  end
+
   it "renders with custom items" do
     render_component(
       legend_text: "What is your date of birth?",