feat: ListenerProvider namespace and interface

Creates a forwards-compatibility shim for the
`ListenerProviderInterface`, in a new subnamespace,
`Zend\EventManager\ListenerProviderInterface`.

Author: weierophinney

TODO-PSR-14.md

@@ -10,8 +10,8 @@
   - [x] Modify `EventManager` internals to use the PSR-14 method if available
   - [x] Mark `StoppableEventInterface` as deprecated
 - [ ] Listener provider implementation
-  - [ ] Create a `ListenerProvider` subnamespace
-  - [ ] Create a `ListenerProviderInterface` shim
+  - [x] Create a `ListenerProvider` subnamespace
+  - [x] Create a `ListenerProviderInterface` shim
   - [ ] Create a `PrioritizedListenerProvider` interface extending the
       `ListenerProviderInterface` and defining a
       `getListenersForEventByPriority($event, array $identifiers = []) : array<int, callable[]>` method.

src/ListenerProvider/ListenerProviderInterface.php

@@ -0,0 +1,21 @@
+<?php
+/**
+ * @see       https://github.com/zendframework/zend-eventmanager for the canonical source repository
+ * @copyright Copyright (c) 2019 Zend Technologies USA Inc. (https://www.zend.com)
+ * @license   https://github.com/zendframework/zend-eventmanager/blob/master/LICENSE.md New BSD License
+ */
+
+namespace Zend\EventManager\ListenerProvider;
+
+/**
+ * @deprecated This interface is a forwards-compatibility shim for use until we can
+ *     provide full PSR-14 compatibility, and will be removed in version 4.0.
+ */
+interface ListenerProviderInterface
+{
+    /**
+     * @param object $event The event for which to retrieve listeners.
+     * @return callable[] Iterable list of listeners to which to pass the event.
+     */
+    public function getListenersForEvent($event);
+}