Adds template annotations turning the `PromiseInterface` into a generic.
Variables `$p1` and `$p2` in the following code example both are
`PromiseInterface<int|string>`.
```php
$f = function (): int|string {
    return time() % 2 ? 'string' : time();
};
/**
 * @return PromiseInterface<int|string>
 */
$fp = function (): PromiseInterface {
    return resolve(time() % 2 ? 'string' : time());
};
$p1 = resolve($f());
$p2 = $fp();
```
When calling `then` on `$p1` or `$p2`, PHPStan understand that function
`$f1` is type hinting its parameter fine, but `$f2` will throw during
runtime:
```php
$p2->then(static function (int|string $a) {});
$p2->then(static function (bool $a) {});
```
Builds on top of reactphp#246 and
reactphp#188 and is a requirement for
reactphp/async#40