build(flake): add lua-language-server to devShell

Problem: lua-language-server is not available in the dev shell, making
it impossible to run local type-checking diagnostics.

Solution: add lua-language-server to the devShell packages.

flake.nix

@@ -65,6 +65,7 @@
           packages = with (pkgsFor system); [
             uv
             python312
+            lua-language-server
           ];
         };
       });