lib: put node:bench behind an --experimental-bench flag

[jasnell]

Stacks after #65919 .. the first commit here is from that

The second commit puts the experimental node:bench properly behind the --experimental-bench flag, which I completely missed on the initial landing.

[nodejs-github-bot]

Review requested:

• @nodejs/config
• @nodejs/loaders
• @nodejs/performance
• @nodejs/startup

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77225/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77327/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (0de4fcc) to head (2f039f1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65920      +/-   ##
==========================================
+ Coverage   90.17%   90.18%   +0.01%     
==========================================
  Files         771      771              
  Lines      265470   265510      +40     
  Branches    50463    50463              
==========================================
+ Hits       239383   239455      +72     
+ Misses      17055    17006      -49     
- Partials     9032     9049      +17     

Files with missing lines	Coverage Δ
lib/internal/bench_runner/cli.js	94.14% <100.00%> (+0.02%)	⬆️
lib/internal/bootstrap/realm.js	96.99% <100.00%> (+0.01%)	⬆️
lib/internal/process/pre_execution.js	97.41% <100.00%> (+0.03%)	⬆️
src/node_builtins.cc	77.66% <100.00%> (+0.07%)	⬆️
src/node_options.cc	80.46% <100.00%> (+0.29%)	⬆️
src/node_options.h	95.61% <100.00%> (+0.03%)	⬆️

... and 29 files with indirect coverage changes

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