Fix breadthfirst layout to center all rows vertically and each row horizontally; and to respect a given bounding box

[soetji]

Associated issues:

• breadthfirst layout doesn't center vertically #3288
• breadthfirst layout doesn't center row #3289
• breadthfirst layout doesn't respect boundingBox #3290

**Notes

• This PR fixes all the issues above from:

Screen.Recording.2024-11-06.at.10.06.17.PM.mov

• This PR fixes all the issues above to:

Screen.Recording.2024-11-06.at.10.07.49.PM.mov

• This PR updates let variable declarations to const, when appropriate, in src/extensions/layout/breadthfirst.js

Checklist

Author:

• The proper base branch has been selected. New features go on unstable. Bug-fix patches can go on either unstable or master.
• Automated tests have been included in this pull request, if possible, for the new feature(s) or bug fix. Check this box if tests are not pragmatically possible (e.g. rendering features could include screenshots or videos instead of automated tests).
• The associated GitHub issues are included (above).
• Notes have been included (above).

Reviewers:

• All automated checks are passing (green check next to latest commit).
• At least one reviewer has signed off on the pull request.
• For bug fixes: Just after this pull request is merged, it should be applied to both the master branch and the unstable branch. Normally, this just requires cherry-picking the corresponding merge commit from master to unstable -- or vice versa.

[mikekucera]

This will get released in 3.31.0
Thanks!