Highlight currently focused/viewed file in file tree

What does this MR do?

This MR is based on issue #24629 (closed)

It can be considered a utterly MVC to get this update underway. The merge attempts to get the ball rolling for the change with a method that removes the highlighted class when the file name has been clicked based on the previous work done here: gitlab-foss!29030 (closed)

Further work needed:

• Update file tree via the IntersectionObserver API as suggested here: gitlab-foss!29030 (comment 177129131)
• Finish removing CSS from the file_row.vue component in favour of more utility classes! This though can be a follow up 😄

Screenshots

No Files Clicked	Some files clicked	All files clicked

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team