Allow any return type from listener functions

[ahocevar]

Fixes #12508.

With the current event listener types, something like this would not pass the type checks:

map.once('rendercomplete', async () => {
  await somethingDone();
  doSomething();
});

This is because the listener function is expected to return a boolean value or void.

In practice, we don't really care about the return value. Returning false from a listener function is equivalent to calling stopPropagation() on the event, but any other return type works too.

This pull request changes the expected return type to any, so the above snippet will now pass type checks.