Add option to allow users to pass autocomplate for the lang-json

[LinuxSuRen]

I already did it in my project. I'd like to contribute it to https://github.com/codemirror/lang-json if there are some developers who believe it's worth it.

In my case, users will type the golang template function name. I hope the autocompletion could help my users. So they don't need to remember or check the function names.

[marijnh]

This looks like it is too much of a special-purpose feature to be added to lang-json. Since it can easily be implemented as an extension outside of the language package, I would suggest you distribute it as your own package.