Add support for installing vscode extensions via brew

[fcjr]

adds a vscode option to the homebrew config to enable installing vscode extensions via brew bundle.

Usage

homebrew = {
    enable = true;
    casks = [ "visual-studio-code" ];
    vscode = [
      "golang.go" # <- Installs the go extension
    ];
  };

Edit:
Just realized this closes #1027, but @ian-h-chamberlain brings up a good question, is the automatic adding of visual-studio-code the best way for this to work? or should I just omit that functionality and document that it is required to be installed or activation will fail?