Core: Mock channel if not present

[yannbf]

Issue: N/A

What I did

Currently when the channel is accessed via addons outside of a browser environment (e.g. tests with testing-react), the following error is thrown:

Error: Uncaught [Error: Accessing non-existent addons channel, see https://storybook.js.org/basics/faq/#why-is-there-no-addons-channel]

This PR sets a mock automatically so users don't have to. I'm not sure about all implications of this change so @tmeasday I'd love if you could check this out!

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

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[tmeasday]

Seems like a decent solution to me @yannbf. Not sure if this would be considered a breaking change (I vote no!), but it certainly may have implications we aren't aware of. I say we find out what they are!

[shilman]

Scary! LGTM 😅