[7.x] Expand Mailable interface

 - expand Mailable interface since `PendingMail::fill()` used some not existed method from the Mailable interface now. And it will caused an fatal error.
 Base PR: laravel#30088

Author: TBlindaruk

src/Illuminate/Contracts/Mail/Mailable.php

@@ -30,4 +30,39 @@ public function queue(Queue $queue);
      * @return mixed
      */
     public function later($delay, Queue $queue);
+
+    /**
+     * Set the recipients of the message.
+     *
+     * @param  object|array|string  $address
+     * @param  string|null  $name
+     * @return self
+     */
+    public function cc($address, $name = null);
+
+    /**
+     * Set the recipients of the message.
+     *
+     * @param  object|array|string  $address
+     * @param  string|null  $name
+     * @return $this
+     */
+    public function bcc($address, $name = null);
+
+    /**
+     * Set the recipients of the message.
+     *
+     * @param  object|array|string  $address
+     * @param  string|null  $name
+     * @return $this
+     */
+    public function to($address, $name = null);
+
+    /**
+     * Set the locale of the message.
+     *
+     * @param  string  $locale
+     * @return $this
+     */
+    public function locale($locale);
 }

src/Illuminate/Mail/PendingMail.php

@@ -177,7 +177,7 @@ protected function fill(MailableContract $mailable)
     {
         return tap($mailable->to($this->to)
             ->cc($this->cc)
-            ->bcc($this->bcc), function ($mailable) {
+            ->bcc($this->bcc), function (MailableContract $mailable) {
                 if ($this->locale) {
                     $mailable->locale($this->locale);
                 }