fix: allow callbacks to return a promise

Description

this is a type-only change that allows callbacks to return a promise. this was already possible before migrating the codebase to TypeScript, but it wasn't reflected correctly in our typings.

Related Issue

N/A

Motivation and Context

while working on the @nestjs/schedule PR to upgrade to cron@3.x, I got a SonarLint warning about passing an async function to onTick.

How Has This Been Tested?

added test case with async onTick.

Screenshots (if appropriate):

Types of changes

[x] 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:

[x] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly.
[x] I have added tests to cover my changes.
[x] 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).

kelektiv / node-cron

fix: allow callbacks to return a promise #733