Add Visual CI Editor Feature Flag

What does this MR do and why?

Add Visual CI Editor Feature Flag

This is the groundwork for the Frontend Island that will run with Vue 3

References

#580317+

Screenshots or screen recordings

Vue-Flow and some visuals will come in Draft: Initial scaffolding for the Visual CI Ed... (!212169)

How to set up and validate locally

1. Enable the visual_ci_editor feature flag

echo "Feature.enable(:visual_ci_editor)" | gdk rails c

2. Open the GDK on any project
3. Go to the Build entry on the Sidebar and click Visual CI Editor

MR acceptance checklist

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