feat: add isCronTimeValid function to validate cron expressions

[WillianAgostini]

Description

This PR adds a function isCronTimeValid to validate CRON expressions without using try/catch blocks.

Related Issue

Issue #958

Motivation and Context

Makes it easier to validate CRON expressions

How Has This Been Tested?

Added two new test cases

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• If my change introduces a breaking change, I have added a ! after the type/scope in the title (see the Conventional Commits standard).

[sheerlox]

Works for me, thanks for the PR! @intcreator what's your take on adding this feature?

[sheerlox]

Since the parsing function returns the reason why a cron time would be invalid, it could be interesting to find a way to return the error.

[WillianAgostini]

Good point! I see two possible approaches here:

1. Returning an object with valid and error and renaming function to validateCronExpression:

{
  valid: boolean;
  error?: Error;
}

2. Using a negative approach where the function returns null if valid expression and an error object otherwise and renaming the function to isCronExpressionInvalid:

const error = isCronExpressionInvalid('not valid');
if (error) {
  // Handle the error
}

Which one do you think fits best with the project's style?

[sheerlox]

I think the first approach with a function named validateCronExpression works best. Since the _parse function is private, this should be added as a static function to CronTime and re-exported from index.ts.

P.S. probably wait for @intcreator's feedback just in case!

[intcreator]

I agree with that approach @sheerlox