Add basic template annotations

This adds basic type safety annotations for static analyzers like PHPStan and Psalm. This will cover around 80% of the use cases and a follow-up PR for all supported versions will be proposed later to get it to a 100% of close to a 100%.

By adding these annotations methods returning a promise can hint their resolving type by adding `@return PromiseInterface<bool>` when they for example resolve to a boolean. By doing that Psalm and PHPStan will understand that the following bit of code will not become an issue because the method's contract promised a boolean through the promise:

```php
$promise->then(static function (bool $isEnabled) {});
```

However, the following will yield errors:

```php
$promise->then(static function (string $isEnabled) {});
```

This PR is a requirement for reactphp/async#40

Author: WyriHaximus

src/React/Promise/PromiseInterface.php

@@ -2,7 +2,13 @@
 
 namespace React\Promise;
 
+/** @template T */
 interface PromiseInterface
 {
+    /**
+     * @template TReturn of mixed
+     * @param callable(T): TReturn $fulfilledHandler
+     * @return (TReturn is PromiseInterface ? TReturn : PromiseInterface<TReturn>)
+     */
     public function then($fulfilledHandler = null, $errorHandler = null, $progressHandler = null);
 }