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

.gitignore

@@ -41,3 +41,4 @@ packagecache
 *.exe
 *.out
 *.app
+/.direnv/