Substitute environment variables into config paths

[KarelPeeters]

Fixes #174.

I've tested this PR in the VSCode plugin using the Language Server User Path, and it works great for me.

[KarelPeeters]

I'm not sure what the best way to handle non-existing environment variables is. Right now I just assume a default value of "" like bash without set -u, but maybe another option is better:

• explicitly show an error to the user
• silently ignore any paths that contain an undefined variable. This would also work as a convenient way to have optional libraries in the config file.

[Schottkyc137]

Thanks a lot for your contribution! This is a feature that many users will like.

I have added a few comments. None of them are blocking, so you may choose to address them but you can also ignore them. Just resolve the conversation so I know when you are ready.

[KarelPeeters]

I think I've addressed everything, let me know if there's anything else to improve!

[Xcodo]

Thanks for doing this PR 👍. This was a problem for my team this week, fantastic to see a fix I can roll out soon!

[KarelPeeters]

Thanks for the quick feedback and merge!