Add project_runners_vue_ui feature flag

What does this MR do and why?

This change adds a new feature flag to show or hide the new project runners UI while hiding the old version of the UI.

This first addition shows an empty space to add the feature.

Note: This is a first step towards #33803, more MRs to come.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the project_runners_vue_ui feature flag
2. In a project, visit Settings > CI/CD
3. Open the Runners section
4. Stare into the empty nothingness of this in progress feature.

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 #33803