feat: remove parser interface and return default structs for parser and schedule

[janrnc]

Refactor the core API to improve decoupling between schedule parsing and job execution by replacing the Parser interface with concrete structs and restructuring the schedule-related code.

Changes

• Refactor Core API:
  • Remove cron.Add(spec, cmd) method; require all jobs use cron.Schedule(schedule, cmd).
  • Replace Parser interface with concrete DefaultParser struct.
  • Expose DefaultParser and DefaultSchedule as public types.
  • Update NewDefaultParser to return (*DefaultParser, error).
  • Remove WithParser and WithSeconds configuration options.
• Restructure Schedule Code:
  • Rename spec.go to schedule.go and spec_test.go to schedule_test.go.
  • Move Schedule interface from cron.go to schedule.go.
• Documentation & Testing:
  • Update documentation and all existing tests to reflect the new workflow of parsing schedules independently before adding them to the scheduler.

See #49

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.64%. Comparing base (c214790) to head (4330c4c).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
parser.go	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   95.22%   94.64%   -0.59%     
==========================================
  Files          18       18              
  Lines         901     1083     +182     
==========================================
+ Hits          858     1025     +167     
- Misses         30       45      +15     
  Partials       13       13              

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[github-actions]

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