DEV: Move calendar date + time picker from local dates into core component (#23023)

This commit moves the calendar date and time picker shown in
the local dates modal into a core component that can be reused
in other places. Also add system specs to make sure there isn't
any breakages with this feature, and a section to the styleguide.

plugins/discourse-local-dates/spec/system/page_objects/modals/insert_date_time.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Modals
+    class InsertDateTime < PageObjects::Modals::Base
+      MODAL_CSS_CLASS = ".discourse-local-dates-create-modal"
+
+      def calendar_date_time_picker
+        @calendar_date_time_picker ||=
+          PageObjects::Components::CalendarDateTimePicker.new(MODAL_CSS_CLASS)
+      end
+
+      def select_to
+        find(".date-time-control.to").click
+      end
+
+      def select_from
+        find(".date-time-control.from").click
+      end
+
+      def delete_to
+        find(".delete-to-date").click
+      end
+    end
+  end
+end