fix(nix): fix shell.nix for non flake nix-shell invocation

- relocated shell.nix to nix/shell.nix
- initialized shell.nix at root to correctly invoke the shell with
  `nix-shell` command for non flake

nix/shell.nix

@@ -0,0 +1,31 @@
+{
+  alejandra,
+  statix,
+  deadnix,
+  shfmt,
+  shellcheck,
+  jsonfmt,
+  lefthook,
+  kdePackages,
+  mkShellNoCC,
+}:
+mkShellNoCC {
+  #it's faster than mkDerivation / mkShell
+  packages = [
+    # nix
+    alejandra # formatter
+    statix # linter
+    deadnix # linter
+
+    # shell
+    shfmt # formatter
+    shellcheck # linter
+
+    # json
+    jsonfmt # formatter
+
+    # CoC
+    lefthook # githooks
+    kdePackages.qtdeclarative # qmlfmt, qmllint, qmlls and etc; Qt6
+  ];
+}