feat: array value completion from contributions

[leonrinkel]

This PR enables JSONWorkerContribution to contribute array value completions. I originally requested this feature in #263 and have now taken a first stab at implementing it. To avoid changing the API, I reused collectValueCompletions, but it doesn't feel like the best fit for this use case. Another approach might be introducing something like collectArrayCompletions, possibly with better fitting attributes like the index the cursor is at. I'm happy to hear your thoughts and feedback!

[leonrinkel]

@microsoft-github-policy-service agree

[aeschli]

Thanks for looking at this!
I agree that it would be better to add collectArrayCompletions as an new (optional) callback.
It might look like that:
collectArrayCompletions(uri: string, location: JSONPath, index: number, result: CompletionsCollector): PromiseLike<any>