Improve Vue dropdown filter results on New Project page

What does this MR do and why?

This MR improves the Vue dropdown filter results on New Project page, previously added in !68778 (merged).

• Hide the Groups and Users sections when there are no matches
• Show No matches found when there are no matches
• Automatically focus on the input when the dropdown opens

This change is behind the feature flag paginatable_namespace_drop_down_for_project_creation, defaulted to off.

Original issue: #26732 (closed)

Screenshots or screen recordings

before	after

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the paginatable_namespace_drop_down_for_project_creation feature flag

   Feature.enable(:paginatable_namespace_drop_down_for_project_creation)

2. Visit the new project page such as http://127.0.0.1:3000/projects/new

3. Test the changes

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 #26732 (closed)