feat: get betterSetInterval ready for 2025

[fnesveda]

Historically, functions scheduled with betterSetInterval needed to accept a callback and then call it when done, to schedule their next invocation. This was because Promises were not a thing in 2015 when this function was originally written. It's silly to have this requirement now, almost ten years later, when async/await and Promises are a normal feature.

This change adds support for scheduling any function, sync or async, accepting or not accepting a callback, and having it just work.

betterSetInterval(async () => {
    await myAsyncOperation();
}, 1000);

For backwards compatibility reasons, a dummy callback is still passed to the invoked function as its first argument, otherwise the functions that don't expect this change would break.