refactor: use generics to improve type declarations for async/if then

[prajwalkulkarni]

Resolves #1096

Description

What is the purpose of this pull request?

This pull request:

• Adds generics for improved type safety for the ifthenAsync function.
• Uses generic parameter defaults for optional generic types U & V.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves [RFC]: improve type declarations for @stdlib/utils/async/if-then #1096

Questions

Any questions for reviewers of this pull request?

No.

Other

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No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[stdlib-bot]

👋 Hi there! 👋

And thank you for opening your first pull request! We will review it shortly. 🏃 💨

[kgryte]

@Planeshifter Would you mind reviewing this?

[Planeshifter]

@prajwalkulkarni Thanks for submitting this PR. LGTM!