Modernize more of the transform functions

[mstange]

Two changes:

• Use createStackTableBySkippingDiscarded in focusSubtree, focusFunction, and focusCategory.
• Change mergeCallNode to only create a new prefix column, like mergeFunction.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.45%. Comparing base (23f14bf) to head (bc32214).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5934   +/-   ##
=======================================
  Coverage   85.44%   85.45%           
=======================================
  Files         321      321           
  Lines       32164    32068   -96     
  Branches     8781     8829   +48     
=======================================
- Hits        27483    27403   -80     
+ Misses       4245     4234   -11     
+ Partials      436      431    -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.