[3.x] Add template annotations

[WyriHaximus]

Adds template annotations turning the PromiseInterface into a generic.

Variables $p1 and $p2 in the following code example both are PromiseInterface<int|string>.

$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:

$p2->then(static function (int|string $a) {});
$p2->then(static function (bool $a) {});

Builds on top of #246 and #188 and is a requirement for reactphp/async#40

[clue]

@WyriHaximus Solid progress! 👍

For context: We've had a call together with @WyriHaximus and @SimonFrings earlier today, discussing our progress for template annotations and unhandled rejections. We've discussed most of these points internally already, but figured it makes sense to post here as well for transparency.

[clue]

@WyriHaximus Went over your changes together with @SimonFrings and addressed all outstanding issues, great work, now let's get this shipped!

[WyriHaximus]

@clue @SimonFrings Thank you for picking up and finishing this off and having my back in these challenging personal times ❤️ .

[clue]

@WyriHaximus No problem, we're here! Happy to support you during tough times. Take care and stay strong!