src: allow empty --experimental-config-file

[marco-ippolito]

This PR makes --experimental-config-file accept no arg.
When no arg is passed, it will default to node.config.json.
--experimental-default-config-file is just an alias of --experimental-config-file.
The reason for this change is so when stable we can just remove --experimental-default-config-file and rename --experimental-config-file to --config-file.
Users can execute node with node --config-file index.js or node --config-file=foo.json index.js.

[nodejs-github-bot]

Review requested:

• @nodejs/config

[pmarchini]

LGTM

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71151/

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.75%. Comparing base (f6464c5) to head (e53ba12).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_config_file.cc	96.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61610      +/-   ##
==========================================
- Coverage   89.76%   89.75%   -0.01%     
==========================================
  Files         673      674       +1     
  Lines      203944   204404     +460     
  Branches    39191    39275      +84     
==========================================
+ Hits       183080   183473     +393     
- Misses      13194    13236      +42     
- Partials     7670     7695      +25     

Files with missing lines	Coverage Δ
lib/internal/process/pre_execution.js	95.84% <100.00%> (-0.13%)	⬇️
src/node.cc	76.28% <ø> (+0.19%)	⬆️
src/node_config_file.h	100.00% <ø> (ø)
src/node_options.cc	76.30% <100.00%> (+0.02%)	⬆️
src/node_options.h	97.88% <ø> (-0.02%)	⬇️
src/node_config_file.cc	83.64% <96.00%> (+1.69%)	⬆️

... and 51 files with indirect coverage changes

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[pmarchini]

Why are we removing those entries in the test?