use more exact return type in Notification#do

[jokester]

Description:

This prevents a return type of any when the type can be inferred from callback types.

// Before: return type is always `any`

// ============

// After:

// when all 3 callbacks are specified: return type becomes (union of their return types)
const a = notification.do(
  _ => 'str',
  _ => 123,
  _ => ({a: 1})
);  // => a: string | number | {a: number}

// when not all of callbacks are specified:
// return type becomes (undefined | union of specified return types)
const b = notification.do(
  _ => 'str'
); // => b: string | undefined

Related issue (if exists):

[benlesh]

Closing this in favor of #5478

Thank you for your effort, @jokester ... it's helpful and it drew our attention to the issue.