Export parseCronString to allow third party use

[idpaterson]

🤔 This is a ...

• New feature
• Bug fix
• Demo update
• Component style update
• TypeScript definition update
• Bundle size optimization
• Performance optimization
• Refactoring
• Code style optimization
• Test Case
• README update
• Other (export existing code)

🔗 Related issue link

Simplifies my pull request to add a feature to backrest, a project that uses react-js-cron.

💡 Background and solution

The output of parseCronString() is useful for me but currently the only way to get it is through setValuesFromCronString() which would be very cumbersome.

This change simply exports the parseCronString() function with a note in the README. This will allow backrest to avoid an additional cron parsing dependency or the ugly setValuesFromCronString() code linked above.

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• Demo in storybook is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Tests are updated and passed without a decrease in coverage
• Format (lint & prettier) script passed
• Build script is working
• README API section is updated or not needed

[xrutayisire]

Thank you for the PR, doing a new release now 🚀

[xrutayisire]

Released in https://github.com/xrutayisire/react-js-cron/releases/tag/v5.2.0 🚀

[idpaterson]

Thank you!