feat: add nix flake

[elsirion]

Adds a nix flake with:

1. Default package that builds the mcp-language-server binary
   • Can be run using nix run github:elsirion/mcp-language-server?ref=nix or nix run github:isaacphi/mcp-language-server once merged
   • Makes it easy to add to nix-based system configs
2. Developer shell that can be entered using nix develop if one doesn't have go installed globally