revert: re-instate extension of Throwable in supplied exception interfaces

See https://github.com/php-fig/container/pull/32#issuecomment-943081411

src/ContainerExceptionInterface.php

@@ -2,9 +2,11 @@
 
 namespace Psr\Container;
 
+use Throwable;
+
 /**
  * Base interface representing a generic exception in a container.
  */
-interface ContainerExceptionInterface
+interface ContainerExceptionInterface extends Throwable
 {
 }