feat: add support for @hubot-friends/hubot-slack adapter

[KeesCBakker]

The old Slack Hubot adapter is not maintained. Fortunately there is a modern forked version with support for Socket Mode.

The PR implements #156

I'm not sure what testing can be done here, as the adapter should still behave in the same way. I'm still checking if this is locally the case.

[stephenyeargin]

@KeesCBakker Code changes look fine and shouldn't disrupt any existing installs. Did you want to do more testing on your end before we merge it in, or is it good to go?

[KeesCBakker]

I've tested it, works fine. I have no clue on how to test it further out in the open. If the adapter keeps working fine I think it will be the new standard for Hubot and Slack.

[stephenyeargin]

Reading the linked PR from #156, I see that v7 is likely to break the existing implementation anyway if the HUBOT_ADAPTER / adapterName has to include the full package name at load time. Solution is likely to add a handful more cases or a helper method to de-reference the adapterName from the implementation.

[KeesCBakker]

Yeah, and some of them will be prefixed by hubot-. Many developments there, but let's see how far the users of this package get. I cannot imagine ppl moving towards v7 fast.

[stephenyeargin]

Attempting to address it in #160.