Refactor scripts-dev/complement.sh logic to avoid exit (#19209)

This is useful so that the script can be sourced by other scripts
without exiting the calling subshell (composable).

This is split out from #19208
to make easy to understand PR's and build up to where we want to go.

Author: MadLittleMods

changelog.d/19209.misc

@@ -0,0 +1 @@
+Refactor `scripts-dev/complement.sh` logic to avoid `exit` to facilitate being able to source it from other scripts (composable).