watch: fix interaction with multiple env files

[marco-ippolito]

Fixes: #60599

The issue was that:

  std::string env_file;
  std::string optional_env_file;

were marked as a string while multiple values were supported.
I added a test to make sure we dont support comma separated values

[nodejs-github-bot]

Review requested:

• @nodejs/config

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.54%. Comparing base (e72761f) to head (2fe75ac).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60605      +/-   ##
==========================================
- Coverage   88.57%   88.54%   -0.04%     
==========================================
  Files         704      704              
  Lines      208087   208087              
  Branches    40090    40084       -6     
==========================================
- Hits       184306   184243      -63     
- Misses      15849    15882      +33     
- Partials     7932     7962      +30     

Files with missing lines	Coverage Δ
src/node_options.h	97.86% <ø> (ø)

... and 51 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70071/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70073/