Schema autocomplete support

[nyurik]

Hi, not sure if you saw it, but now Kibana (Elasticsearch UI) uses HJSON for the Vega visualizations! And almost immediately I have started receiving requests to add autocomplete support to the ACE editor. The most straightforward (IMO) way to do it would be to use a JSON schema - and simply look up allowed values at each point. Can you think of a good way to add that to HJSON? I saw a PR (a bit stale) for the ACE's JSON autocomplete using Microsoft's intelisense library - I wounder if we can use the same approach? https://github.com/ajaxorg/ace/pull/2885/files

[alexsb]

This would be quite helpful! Bump!