fix: sed invocations to work out of the box on macOS (#615)

* fix: sed invocations to work out of the box on macOS

These fixes make the sed invocations run on macOS but I am not sure if they will break the GNU sed invocations in return…

* fix: add macOS branches to the shell scripts

The sed command line was difficult (impossible?) to be made portable so I just added macOS conditions.

* Prototype the solution using an alias

It seems that this will not work because I keep getting "sedi: command not found".

* fix: use a Bash function instead of an alias to run sed portably

This seems to work on my local so let's see if it runs in the CI on GitHub.

* fix: use 2 spaces like the original scripts did

Not sure why VS Code went with 4.

* fix: use sedi for the build-changelog `1i\` part as well

I am curious if Ubuntu will handle the newline…