Add "directory_code_dropdown_updates" feature flag

What does this MR do and why?

• Adds a new feature flag directory_code_dropdown_updates that flags a new code dropdown component
• To keep this MR in scope, only added the first section in the new component

References

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• first part #450667 (closed)

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desktop
mobile discussion on reusing the same component for mobile

How to set up and validate locally

1. Enable directory_code_dropdown_updates and blob_repository_vue_header_app feature flag

   Feature.enable(:directory_code_dropdown_updates)
   Feature.enable(:blob_repository_vue_header_app)

1. Go to a directory
2. Click on Code

Related to #450667 (closed) #513599 (closed)