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 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.69%. Comparing base (3f52482) to head (4bf9c84).

Files with missing lines	Patch %	Lines
src/node.cc	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61610      +/-   ##
==========================================
- Coverage   91.55%   89.69%   -1.87%     
==========================================
  Files         356      706     +350     
  Lines      149601   218143   +68542     
  Branches    23395    41734   +18339     
==========================================
+ Hits       136967   195659   +58692     
- Misses      12371    14412    +2041     
- Partials      263     8072    +7809     

Files with missing lines	Coverage Δ
lib/internal/process/pre_execution.js	98.38% <100.00%> (+14.63%)	⬆️
lib/internal/test_runner/runner.js	93.67% <100.00%> (+0.31%)	⬆️
src/node_config_file.cc	83.73% <100.00%> (ø)
src/node_config_file.h	100.00% <100.00%> (ø)
src/node_options.cc	76.61% <100.00%> (ø)
src/node_options.h	98.00% <ø> (ø)
src/node.cc	76.90% <87.50%> (ø)

... and 465 files with indirect coverage changes

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[github-actions]

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[github-actions]

Failed to start CI

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✔  Jenkins credentials valid
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[SyntaxError: Unexpected token '<', ..."    
  https://github.com/nodejs/node/actions/runs/24461444173

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/72703/