Add devshell and initial bats tests

flake.nix

@@ -15,6 +15,19 @@
       in
       {
         packages.default = pkgs.callPackage ./default.nix { };
+        devShells.default = pkgs.mkShell {
+          packages = [
+            (pkgs.bats.withLibraries (p: [
+              p.bats-assert
+              p.bats-support
+              p.bats-file
+            ]))
+            pkgs.python3
+            pkgs.python3Packages.flake8
+            pkgs.shellcheck
+            pkgs.shfmt
+          ];
+        };
       }
     );
 }