xargs: accept hyphenated -I and -E values

[kevinburke]

GNU xargs treats the arguments to -I, -E, and -e as values even when they begin with a hyphen. Clap was parsing those strings as options instead, so invocations like 'xargs -I -_' failed before xargs could process input.

Allow hyphen-leading values for those options and add regression coverage for replacement and EOF markers that start with '-'.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.76%. Comparing base (2332e46) to head (f390932).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #668   +/-   ##
=======================================
  Coverage   91.76%   91.76%           
=======================================
  Files          31       31           
  Lines        6203     6206    +3     
  Branches      328      328           
=======================================
+ Hits         5692     5695    +3     
  Misses        390      390           
  Partials      121      121           

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