feat: add return type hints

This patch adds a return type hint to `ContainerInterface::has()`, per
the already documented specification, in preparation for a v3 release.
No other methods were eligible, as they specified `mixed`.

See https://www.php-fig.org/blog/2019/10/upgrading-psr-interfaces/

Signed-off-by: Matthew Weier O'Phinney <matthew@weierophinney.net>

composer.json

@@ -21,7 +21,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "2.0.x-dev"
+            "dev-master": "3.0.x-dev"
         }
     }
 }

src/ContainerInterface.php

@@ -32,5 +32,5 @@ public function get(string $id);
      *
      * @return bool
      */
-    public function has(string $id);
+    public function has(string $id): bool;
 }