dev: Added Nix Flake support

- Enables Nix users to get the git version of waybar
- Enables Nix users to develop waybar easily
- Adds a fully reproducible development environment
  - The user only has to install Nix, no other depencencies
  - Automatic dev env on directory entry through .envrc

flake.nix

@@ -0,0 +1,65 @@
+{
+  description = "Highly customizable Wayland bar for Sway and Wlroots based compositors.";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    devshell.url = "github:numtide/devshell";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, flake-utils, devshell, nixpkgs }:
+    let
+      inherit (nixpkgs) lib;
+      genSystems = lib.genAttrs [
+        "x86_64-linux"
+      ];
+
+      pkgsFor = genSystems (system:
+        import nixpkgs {
+          inherit system;
+        });
+
+      mkDate = longDate: (lib.concatStringsSep "-" [
+        (builtins.substring 0 4 longDate)
+        (builtins.substring 4 2 longDate)
+        (builtins.substring 6 2 longDate)
+      ]);
+    in
+    {
+      overlays.default = _: prev: rec {
+        waybar = prev.callPackage ./nix/default.nix {
+          version = "0.9.16" + "+date=" + (mkDate (self.lastModifiedDate or "19700101")) + "_" + (self.shortRev or "dirty");
+        };
+      };
+      packages = genSystems
+        (system:
+          (self.overlays.default null pkgsFor.${system})
+          // {
+            default = self.packages.${system}.waybar;
+          });
+    } //
+    flake-utils.lib.eachDefaultSystem (system: {
+      devShell =
+        let pkgs = import nixpkgs {
+          inherit system;
+
+          overlays = [ devshell.overlay ];
+        };
+        in
+        pkgs.devshell.mkShell {
+          imports = [ "${pkgs.devshell.extraModulesDir}/language/c.nix" ];
+          commands = [
+            {
+              package = pkgs.devshell.cli;
+              help = "Per project developer environments";
+            }
+          ];
+          devshell.packages = with pkgs; [
+            clang-tools
+            gdb
+          ];
+          language.c.libraries = with pkgs; [
+          ];
+        };
+    });
+}