fix: only reject positional args that are entirely numeric

[lazerg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug (v12): spec file stating with a number rejected #6283
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

isNumeric checks its input with parseFloat, which stops at the first character that cannot be part of a number. A spec file whose name starts with a digit, like npx mocha 1-spec.js, therefore counts as a numeric positional and the CLI bails out with Option NaN is unsupported by the mocha cli.

Comparing against Number instead makes the check look at the whole string, so bare numbers such as mocha 123 still error while file names that merely begin with a digit run as before.

Note that #6221 is still in triage rather than labelled accepting prs, so feel free to close this if you want to discuss it first.

[mark-wiemer]

Looks good, let's wait for tests to pass and also backport this to v11

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.08%. Comparing base (505b5e9) to head (43f8c28).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6222      +/-   ##
==========================================
- Coverage   81.93%   74.08%   -7.85%     
==========================================
  Files          64       65       +1     
  Lines        4866     5669     +803     
  Branches     1100     1337     +237     
==========================================
+ Hits         3987     4200     +213     
- Misses        879     1469     +590     

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