Allow to open `.lco` files and integrate `.loc` and `.cls` like normal `\include` tex files

[martin-v]

Thanks for your work, this app is great!

I work with lco files (description). Can you allow opening these files?

Also it would be awesome if .lco and .cls files would be also listed in the sidebar like it happens in the \include case.

The logic should be something like:

1. look for \documentclass 1.1. if they are [] params, split and look for each param if there is a file called $parm.loc 1.2. check if there is a file named like the documentclass, e.g. \documentclass{developercv} -> developercv.cls
2. look for \LoadLetterOption and matching files, e.g. \LoadLetterOption{myletterconfig} -> myletterconfig.loc

For simpler implementation I would only look up relative path names or even simpler files in the same folder. Also 1.1. could be skiped because it's with 2. exchangeable.

I think using the default editor/completion is better than no completion, most of the normal syntax is used in lco/cls files.