Made the hardcoded completion field limit configurable

[halflifefan]

I'm aware that 4.0.0 is being worked on, couldn't find this exact limit there. Haven't seen public statements or ETAs regarding 4.0.0 either. Don't know whether this change will be applied to the version used in Visual Studio Code but I'm doing this under the assumption that it will considering I found this in the master branch.

Basically, I hit the limit (while doing somewhat reasonable stuff) and I guess this would be a good temp fix. I'm assuming changes to locale/setting.lua and doc/config.md are needed, however I'm not sure what I should've done, make a description and translate to all available languages?