feat(flake): complete revamp with devenv/direnv

Not perfect, and mostly a port of the previous edition. Hours have
already been sunk into this, and need to get this out for consumption,
and for ironing out.
For more information see: https://devenv.sh

NOTE: impure is used only because of the devenv-managed state, do not be
alarmed!

.envrc

@@ -0,0 +1,10 @@
+if ! has nix_direnv_version || ! nix_direnv_version 2.2.1; then
+    source_url 'https://raw.githubusercontent.com/nix-community/nix-direnv/2.2.1/direnvrc' 'sha256-zelF0vLbEl5uaqrfIzbgNzJWGmLzCmYAkInj/LNxvKs='
+fi
+
+watch_file flake.nix
+watch_file flake.lock
+
+if ! use flake . --impure; then
+    printf '%s\n' "devenv could not be built. The devenv environment was not loaded. Make the necessary changes to devenv.nix and hit enter to try again." >&2
+fi