port argument in App can be ignored when using a custom receiver

[seratch]

Description

As described at https://github.com/slackapi/bolt-js/issues/1344 , the port argument in App constructor can be silently ignored if the App accepts receiver argument as well. Bolt should ensure the App's port parameter is passed on to the receiver.

What type of issue is this? (place an x in one of the [ ])

• [ ] bug
• [x] enhancement (feature request)
• [ ] question
• [ ] documentation related
• [ ] example code related
• [ ] testing related
• [ ] discussion

Requirements (place an x in each of the [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.