Add support for `nixd` LSP

**Name:** nixd
**URL:** https://github.com/nix-community/nixd

A feature-rich nix language server interoperating with C++ nix.

Some notable features provided by linking with the Nix library include:

- Nixpkgs option support, for all option system (NixOS/home-manager/flake-parts).
- Nixpkgs package complete, lazily evaluated.
- Shared eval caches (flake, file) with your system's Nix.
- Support for cross-file analysis (goto definition to locations in nixpkgs).
- Inlay hints and semantic tokens (see https://github.com/nix-community/nixd/blob/main/nixd/docs/features.md)