feat(nix): add .direnv

very commonly used to enter Nix devshells (see https://github.com/nix-community/nix-direnv for more info)

community/Nix.gitignore

@@ -1,3 +1,6 @@
 # Ignore build outputs from performing a nix-build or `nix build` command
 result
 result-*
+
+# Ignore automatically generated direnv output
+.direnv