Add registerCompletionHandler to main.d.cts

[sosnovsky]

In 6.1.1 release it's not possible to use registerCompletionHandler in CommonJS project.
This PR adds registerCompletionHandler to main.d.cts, so it can be imported in CommonJS

Fixes #3306.

[novemberborn]

Thank you for the thorough PR @sosnovsky.

The lack of CJS here is intentional. Mostly to avoid polluting the default export, which is shared with ESM. Also because usage ought to be rare enough that you can work around it in CJS using dynamic import.

[sosnovsky]

Got it, thanks for explanation, will try to use it through dynamic import.