Migrate build tool from `tsup` to `tsdown`

Is there an existing issue or pull request for this?

• I have searched the existing issues and pull requests

Feature description

The project currently uses tsup as its build tool. However, tsdown is intended to replace tsup in the future and is actively positioned as its successor.

Desired solution

Migrate the project from tsup to tsdown following the official migration guide:

https://tsdown.dev/guide/migrate-from-tsup

Alternatives considered

• Continuing to use tsup until it is officially deprecated

Additional context

• https://github.com/orhun/git-cliff/pull/1382#issuecomment-3958434105