Nix-VSCode integration

[x-mass]

Currently CPP-related VSCode functionality works randomly. We could add .vscode dir to root, holding paths of where to look for compiler, includes, etc. Refer to DBMS repo for example.
Require nix files modification as well

[ioxid]

I'm working from vscode. I use direnv (extension + .envrc file) to make the compiler (clang in my case) visible in vscode and CMake Tools to configure the project. It works fine, even including clangd, although I had to configure it a bit.