Update doc.json

[AndreasMatthias]

To update an existing doc.json file, one needs to remember the --doc argument with which it was originally created. This can be cumbersome and error-prone.

This pull requests add a new luals.config section to doc.json that stores this--doc argument:

{
    "DOC": "/path/to/my/lua/project/",
    "defines": [],
    "fields": [],
    "name": "LuaLS",
    "type": "luals.config"
}

If this section is available LuaLS can be called with --doc_update to update an existing doc.json.

$ lua-language-server --doc_update

Note: The fields defines and fields in the new JSON section are not necessary. However I've added them, because each and every section has these fields, which makes working with the JSON file a bit easier.

[sumneko]

Thank you!