feat(parser): replace ParseStandard() with StandardOptions

[janrnc]

• Removed the standalone ParseStandard convenience function and the internal standardParser variable from parser.go.
• Added a new exported constant StandardOptions that encapsulates the default standard cron parsing options (Minute | Hour | Dom | Month | Dow | Descriptor).
• Revised doc.go to demonstrate the new recommended pattern: instantiating a default parser using cron.NewDefaultParser(cron.StandardOptions) and calling Parse() on it.
• Updated test files to initialize their own standard parser instances where needed, replacing the old ParseStandard calls.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.14%. Comparing base (a84ff6c) to head (c73cc59).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   94.64%   95.14%   +0.50%     
==========================================
  Files          18       18              
  Lines        1083      886     -197     
==========================================
- Hits         1025      843     -182     
+ Misses         45       30      -15     
  Partials       13       13              

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[github-actions]

🎉 This PR is included in version 0.7.0 🎉

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