feat: Allowing preserving double dashes for variadic args (#229)

[alcroito]

Previously, all double dash tokens were discarded when parsed.

A new preserve option has been added to the double_dash setting for arguments. When set to preserve on a variadic argument, any double dash tokens (--) encountered while parsing will be retained as part of the argument's value.

Just like for non-preserve behavior, enable_flags processing is disabled for preserve behavior as well. That's because any potential common flags like --verbose should be captured inside the variadic arg, rather than being treated as flags.

Added tests to verify the new behavior.

Fixes #229

---

Note

Introduces a new double_dash behavior to retain -- tokens in variadic args and updates parsing, spec, and docs accordingly.

• Adds SpecDoubleDashChoices::Preserve and serializes double_dash when automatic or preserve
• Parser: -- always disables flag parsing; when next arg is variadic with double_dash=preserve, the -- token is kept as an arg value
• Builder: SpecArgBuilder::double_dash(...) to set behavior programmatically
• Docs: updates arg.md with double_dash="preserve" example
• Tests: new cases covering preserved/skipped --, multiple args, and edge cases

^{Written by Cursor Bugbot for commit f1ac35b. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

❌ Patch coverage is 86.42857% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.08%. Comparing base (fc9a6da) to head (f1ac35b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/parse.rs	90.22%	6 Missing and 7 partials ⚠️
lib/src/spec/arg.rs	0.00%	1 Missing and 2 partials ⚠️
lib/src/spec/builder.rs	25.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #417      +/-   ##
==========================================
- Coverage   52.08%   50.08%   -2.01%     
==========================================
  Files          47       47              
  Lines        5894     6100     +206     
  Branches     5894     6100     +206     
==========================================
- Hits         3070     3055      -15     
- Misses       1438     1513      +75     
- Partials     1386     1532     +146     

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✅ Bugbot reviewed your changes and found no bugs!