Add repository first-content User Timings

What does this MR do and why?

Adds User Timings for the first file-tree row and the first highlighted code in the repository view. Rows and code chunks only render when they scroll into view, so the first one is the point I can measure the same way on every visit. Each folder or file visit gets its own timing. The file timing waits for the highlighting, unlike the existing <SourceViewer> render timing.

The full-list, commit and codeowners timings from the issue are in #638419.

References

Related to #402772

Screenshots or screen recordings

No UI change. The new measures are "Repository File Tree: First row" and "Repository File Viewer: First highlighted content".

How to set up and validate locally

1. Record a performance profile while you open a repository folder and then a code file.
2. Both measures show up under Timings.
3. Scroll down, the measures don't change.
4. Open another folder or file and a new measure is recorded for it.

MR acceptance checklist

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