feat: support use vscode variables in settings

[njzydark]

https://github.com/nix-community/nixd/blob/main/nixd/docs/configuration.md#where-to-place-the-configuration

According to the above document, when defining the expr of options, only absolute paths can be used. This is very inconvenient for configuring across machines. Therefore, this PR introduces the vscode-variables package to support using ${workspaceFolder} in configuration files to define paths.

before

{
    "nixd": {
      "options": {
        "nixpkgs": {
          "expr": "import <nixpkgs> { }"
        },
        "nix-darwin": {
          "expr": "(builtins.getFlake \"/absolute/path/to/flake\").darwinConfigurations.default.options"
        }
      }
    }
}

after

{
    "nixd": {
      "options": {
        "nixpkgs": {
          "expr": "import <nixpkgs> { }"
        },
        "nix-darwin": {
          "expr": "(builtins.getFlake \"${workspaceFolder}/flake\").darwinConfigurations.default.options"
        }
      }
    }
}

[jnoortheen]

Thanks for the contribution @njzydark !