src: use starts_with instead of rfind/find

[tniessen]

Using starts_with makes the code more readable than usingrfind() and avoids the inefficiency of the forward search incurred by find().

[nodejs-github-bot]

Review requested:

• @nodejs/startup

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.52%. Comparing base (9bcfbeb) to head (a4324c6).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61426      +/-   ##
==========================================
- Coverage   88.53%   88.52%   -0.01%     
==========================================
  Files         704      704              
  Lines      208883   208883              
  Branches    40333    40335       +2     
==========================================
- Hits       184926   184906      -20     
- Misses      15934    15964      +30     
+ Partials     8023     8013      -10     

Files with missing lines	Coverage Δ
src/node_builtins.cc	76.47% <100.00%> (+0.18%)	⬆️
src/path.cc	72.81% <100.00%> (ø)

... and 35 files with indirect coverage changes

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[Renegade334]

Adding dont-land for pre-C++20.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70884/