Export old code into es6 modules

What does this MR do?

Export old code into es6 modules:

1. app/assets/javascripts/project_find_file.js
2. app/assets/javascripts/search.js
3. app/assets/javascripts/search_autocomplete.js
4. app/assets/javascripts/syntax_highlight.js

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#28869 (closed)