direnvrc: Add nix_direnv_version

In the style of direnv's `direnv_version`.
729fbecd96/stdlib.go (L1173-L1179)

Can be tested for with `has`, e.g. to decide on using `source_url`.

scripts/create-release.sh

@@ -10,14 +10,20 @@ if [[ -z "$version" ]]; then
     echo "USAGE: $0 version" 2>/dev/null
     exit 1
 fi
+[[ $version =~ ^([0-9]+)\.([0-9]+)\.([0-9]+)(-?)([^+]*)(\+?)(.*)$ ]]
+declare -a ver; ver=("${BASH_REMATCH[@]:1}")
 
 if [[ "$(git symbolic-ref --short HEAD)" != "master" ]]; then
     echo "must be on master branch" 2>/dev/null
     exit 1
 fi
 
-sed -i -e "s!nix-direnv/.*/direnvrc!nix-direnv/${version}/direnvrc!" README.md
-git add README.md
+sed -i README.md \
+    -e 's!\(nix-direnv/\).*\(/direnvrc\)!\1'"${version}"'\2!' \
+    -e 's?\( ! nix_direnv_version \)[0-9.]\+\(; \)?\1'"${version}"'\2?'
+sed -i direnvrc \
+    -e 's!\(declare major=\).*\( # UPDATE(nix-direnv version)\)!\1'"${ver[0]@Q} minor=${ver[1]@Q} patch=${ver[2]@Q}"'\2!'
+git add README.md direnvrc
 git commit -m "bump version ${version}"
 git tag -e "${version}"