Add a basic interface for showing which files have been viewed and which file is currently active

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)

Screenshots

On Page Load	After Some Files Are Clicked	File Header With Active State

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