add nix flake (#210)

- add following functionality:
  - nix develop with direnv support
  - nix run and nix shell
  - nix fmt for flake.nix
  - and package overlay for https://github.com/NixOS/nixpkgs/pull/225502

- useful docs
  - https://stackoverflow.com/questions/53272197/how-do-i-override-the-libc-in-a-nix-package-to-be-musl
  - dd3aca2d0b/pkgs/top-level/stage.nix (L136)

- inspired by https://github.com/typst/typst/blob/main/flake.nix

flake.lock

@@ -0,0 +1,59 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1685866647,
+        "narHash": "sha256-4jKguNHY/edLYImB+uL8jKPL/vpfOvMmSlLAGfxSrnY=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "a53a3bec10deef6e1cc1caba5bc60f53b959b1e8",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixpkgs-unstable",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs",
+        "utils": "utils"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    },
+    "utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1685518550,
+        "narHash": "sha256-o2d0KcvaXzTrPRIo0kOLV0/QXHhDQ5DTi+OxcjO8xqY=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "a1720a10a6cfe8234c0e93907ffe81be440f4cef",
+        "type": "github"
+      },
+      "original": {
+        "id": "flake-utils",
+        "type": "indirect"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}