Introduce User Timing API metrics for WebIDE

What does this MR do?

In the course of improving the loading performance of WebIDE, this MR sets the basic metrics for monitoring and measurement of the performance in WebIDE using User Timing API.

The measurement areas:

• Navigation tree
• Loading a file in the editor

References:

• Relevant RFC: gitlab-org/frontend/rfcs#51 (closed)
• "Blueprint" implementation in Snippets: #238972 (closed)

Testing:

• Open a project in WebIDE
• Open the DevTools
• Watch the performance bar entries' log in the console

Screenshots

	Performance Bar output
Default loading a branch
Opening a file in a branch
Loading exact file in a branch

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

References #238973 (closed)