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

.github/workflows/ci.yml

@@ -45,3 +45,16 @@ jobs:
       - run: composer self-update --2.2 # downgrade Composer for HHVM
       - run: hhvm $(which composer) install
       - run: hhvm vendor/bin/phpunit
+
+  static-analysis:
+    name: PHPStan
+    runs-on: ubuntu
+    continue-on-error: true
+    steps:
+      - uses: actions/checkout@v3
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: 8.1
+      - run: composer require phpstan/phpstan
+      - name: Execute type checking
+        run: vendor/bin/phpstan --configuration="phpstan.types.neon.dist"

phpstan.types.neon.dist

@@ -0,0 +1,4 @@
+parameters:
+  paths:
+    - types
+  level: max

src/PromiseInterface.php

@@ -2,6 +2,10 @@
 
 namespace React\Promise;
 
+/**
+ * @template T
+ * @template R
+ */
 interface PromiseInterface
 {
     /**
@@ -32,10 +36,15 @@ interface PromiseInterface
      *      than once.
      *  3. `$onProgress` (deprecated) may be called multiple times.
      *
-     * @param callable|null $onFulfilled
-     * @param callable|null $onRejected
-     * @param callable|null $onProgress This argument is deprecated and should not be used anymore.
-     * @return PromiseInterface
+     * @template TFulfilled of mixed
+     * @template TRejected of mixed
+     * @param (callable(T): (PromiseInterface<TFulfilled>|TFulfilled))|null $onFulfilled
+     * @param (callable(R): (PromiseInterface<TRejected>|TRejected))|null $onRejected
+     * @return PromiseInterface<(
+     *   $onFulfilled is not null
+     *     ? ($onRejected is not null ? TFulfilled|TRejected : TFulfilled)
+     *     : ($onRejected is not null ? TRejected : R)
+     * )>
      */
     public function then(callable $onFulfilled = null, callable $onRejected = null, callable $onProgress = null);
 }

types/PromiseInterface.php

@@ -0,0 +1,9 @@
+<?php
+
+use function PHPStan\Testing\assertType;
+use function React\Promise\resolve;
+
+$passThroughBoolFn = static fn (bool $bool): bool => $bool;
+
+assertType('React\Promise\PromiseInterface<bool>', resolve(true));
+assertType('React\Promise\PromiseInterface<bool>', resolve(true)->then($passThroughBoolFn));