Create placeholder route for group level work items

What does this MR do and why?

Blank page where frontend will mount vue app. We will use this new route in the backend to change the URL generator to point here for group level work items that until this point returned null

How to set up and validate locally

1. Enable the feature flag in Rails console Feature.enable(:namespace_level_work_items)
2. Create a group level work item with the following query at http://127.0.0.1:3000/-/graphql-explorer

   mutation {
      workItemCreate(input: {namespacePath: "flightjs", title: "test group level work item", workItemTypeId: "gid://gitlab/WorkItems::Type/1"}) {
        errors
        workItem {
          id
        }
      }
    }

3. Fetch webUrl for the newly created work item

   {
      group(fullPath: "flightjs") {
        name
        workItems {
          nodes {
            webUrl
            id
            iid
          }
        }
      }
    }

4. Navigating to the value returned in webUrl should show a placeholder page with a single string title in the top of the page

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393240 (closed)