feat(isCron): add cron expression validator

[Dueen]

Summary

• add an isCron validator for common five-field Unix cron expressions
• support lists, ranges, positive steps, month and weekday names, and standard aliases
• optionally accept a leading seconds field with { allow_seconds: true }
• document the supported dialect and add focused valid/invalid coverage

Motivation

Closes #2301.

This follows the discussion in #2301 and the earlier attempt in #2306. The validator defaults to the common five-field Unix format and keeps the six-field leading-seconds form opt-in so it does not silently mix Unix and Quartz cron dialects. Quartz-only tokens such as ?, L, W, and # are intentionally rejected.

Validation

• npm test (build, lint, and 325 passing tests)

References

• Cronie crontab(5)
• Quartz CronExpression

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d4e02ee) to head (58b1d46).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2837   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       115    +1     
  Lines         2598      2662   +64     
  Branches       658       676   +18     
=========================================
+ Hits          2598      2662   +64     

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[Copilot]

Pull request overview

This PR adds a new public validator, isCron, to validator.js for validating common five-field Unix cron expressions (with optional opt-in support for a leading seconds field), and documents/validates the supported dialect.

Changes:

• Added src/lib/isCron.js implementing cron parsing/validation (lists, ranges, positive steps, month/weekday names, and standard @... aliases).
• Exported the new validator via src/index.js so it becomes part of the public API.
• Added focused unit tests and README documentation for the new validator.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
test/validators/isCron.test.js	Adds valid/invalid coverage for 5-field cron, aliases, optional seconds field, and a performance-regression guard.
src/lib/isCron.js	Implements isCron validation logic for Unix-style cron expressions and standard aliases.
src/index.js	Wires isCron into the public validator export surface.
README.md	Documents isCron behavior, supported syntax, and the allow_seconds option.

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