feat: add flake for development environment (#1066)

## What are the changes about?

This PR adds a flake.nix and flake.lock to provide a reproducible development environment for ly using Nix.

The development shell provides the dependencies required to build ly:

- zig
- pam
- xcb

## What existing issue(s) does this resolve?

N/A

## Pre-requisites

- [x] I have tested & confirmed the changes work locally
- [x] I have read and fully adhere to the rules set in the contributing guidelines found in `CONTRIBUTING.md`

Reviewed-on: https://codeberg.org/fairyglade/ly/pulls/1066
Reviewed-by: AnErrupTion <anerruption+codeberg@disroot.org>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1790822859,
+        "narHash": "sha256-69xHQhAeMAD2wDXO7T2pcOZIF9Sga2W+JkmY2a11Ops=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "c59305bab2065cfecc4944690d9eedbb56f3a9fa",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}