Remove events from cucumber-plugin in favor of messages

[mpkorstanje]

🤔 What's the problem you're trying to solve?

We've migrated all the internal formatters to use messages. This means that the events Cucumber emits are no longer used. Additionally we have a gherkin-messages module that helps translate between messages and events that is just taking up space now.

✨ What's your proposed solution?

• Deprecate (in v8.x.x) the events and remove them (in v9.0.0).
• Remove gherkin-messages and use messages directly.

⛏ Have you considered any alternatives or workarounds?

I could leave things as they are but the gherkin-messages are a pointless layer of indirection now.

📚 Any additional context?

Cleaning this up will help refactor Cucumbers internals to be slightly less tangled.

[laeubi]

@mpkorstanje as far as I know the events are used by eclipse-plugin (at least we use gherkin-messages there what seems indicate this?). Could you probably take a quick look and possibly create a new issue that describes what needs to be done to migrate away from this in case of deprecation?