chore: add support for .direnv

- Loads the devshell more conveniently

.envrc

@@ -0,0 +1,11 @@
+#! /bin/sh
+
+# reload when these files change
+watch_file flake.nix
+watch_file flake.lock
+
+{
+  # shell gc root dir
+  mkdir -p "$(direnv_layout_dir)"
+  eval "$(nix print-dev-env --no-update-lock-file --no-write-lock-file --profile $(direnv_layout_dir)/flake-profile)"
+}

.gitignore

@@ -1,3 +1,4 @@
+/.direnv
 /result*
 /target/
 /tarpaulin*