notify when new script version is available

add a version check for the updater itself to fix #15

Signed-off-by: Lucas Larson <LucasLarson@riseup.net>

update

@@ -1,5 +1,5 @@
 #!/usr/bin/env sh
-update='2023-05-22'
+update='2023-05-23'
 IFS="$(command printf -- ' \t\n|')"
 IFS="${IFS%'|'}"
 command printf -- '\n\n                 .___       __\n __ ________   __\174 _\057____ _\057  \174_  ____\n\174  \174  \134____ \134 \057 __ \174\134__  \134\134   __\134\057 __ \134\n\174  \174  \057  \174_\076 \076 \057_\057 \174 \057 __ \134\174  \174 \134  ___\057\n\174____\057\174   __\057\134____ \174\050____  \057__\174  \134___  \076\n      \174__\174        \134\057     \134\057          \134\057\n a Lucas Larson production\n\n' >&2 && command sleep -- 1
@@ -18,6 +18,13 @@ command printf -- '\n\n' >&2
   command printf -- 'No internet connection detected.\nAborting update.\n' >&2
   return "${update:-1}"
 }
+set -- 'https://github.com/LucasLarson/update/raw/HEAD/update'
+command test "$({ command wget --output-document=- --quiet -- "${1-}" || command curl --location --silent -- "${1-}"; } 2>/dev/null | command sed -n -e '$s/[^[:digit:]]//gp')" -le "$(command printf -- '%s\n' "${update-}" | command sed -e 's/[^[:digit:]]//g')" || {
+  command printf -- 'An update to this updater is available\041 Download it from here:\n%s\n' "${1-}" >&2
+  update=''
+  unset -v -- update 2>/dev/null
+  exit
+}
 command printf -- '\360\237\215\272  checking for Homebrew installation...' >&2
 if command -v -- brew >/dev/null 2>&1; then
   command printf -- '  \342\234\205  found\n' >&2