[json] Support extending JSON completion

[WebFreak001]

Right now there are some hard coded completions like npm or bower versions (https://github.com/Microsoft/vscode/tree/master/src/vs/languages/json/common/contributions)

However it seems it isn't extensible with custom packages, so adding an API for that would be great because I would really like to add support for completing versions in my package

[egamma]

Great minds think alike 😄 This is on the plan and we start on it in January, pls see the January iteration plan #1826, in the Debt section.

CC @aeschli

[WebFreak001]

Seems like this has been added in vscode 1.1.0

[aeschli]

Yes, correct. I forgot to close this issue and comment.
The previously hard coded completions like npm or project.json versions have now moved to the JavaScript resp C# extensions.
Each of these extension registers a completion and hover provider for the corresponding JSON files, and uses the new javac-parser node-module to parse the JSON in order to evaluate the current location in the JSON file.
Have a look at dotnet/vscode-csharp#224 to see how this works.