fix: allow callbacks to return a promise

[sheerlox]

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

• 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).

[ncb000gt]

🎉 This PR is included in version 3.1.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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