[5.5] Add to and bcc to mailer contract

[fernandobandeira]

Fixes #18361

#19928 was reverted since it broke MailFake.

However we can add bcc and to, MailFake implements them:
https://github.com/laravel/framework/blob/master/src/Illuminate/Support/Testing/Fakes/MailFake.php#L110

    /**
     * Begin the process of mailing a mailable class instance.
     *
     * @param  mixed  $users
     * @return \Illuminate\Mail\PendingMail
     */
    public function to($users)
    {
        return (new PendingMailFake($this))->to($users);
    }

    /**
     * Begin the process of mailing a mailable class instance.
     *
     * @param  mixed  $users
     * @return \Illuminate\Mail\PendingMail
     */
    public function bcc($users)
    {
        return (new PendingMailFake($this))->bcc($users);
    }