Create aggregated CHANGELOG.md

[perlun]

In f484f87, we added a first changelog entry. Another one will be created shortly. These are useful for populating the GitHub Releases with data.

However, it would also be slightly useful to have an aggregated CHANGELOG.md that can be used to see the full changelog for the project in one place. The way I envision this to be done:

1. Parse all files matching the release-notes/*.md glob.
2. Sort them based on the Semantic Version of the filename (excluding the v prefix), in descending order. (newest first)
3. Write the result to CHANGELOG.md.

For maximum usage, we should also add a new make target which does the following:

• Runs the script above
• Runs git release with the correct parameter for the next release

Doing it like this would ensure we never forget to run this. One challenge could be to remember "what is the next release?". But I guess the simplest way to accomplish that is to just hardwire it in the Makefile for now. We don't create new releases that often, so it's quite fine if it's a manual process.

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Ideally, the script described above would be a Perlang script. But we don't have any SemVer library for Perlang yet... or support for reading files. 😅 So it'll probably be a little Ruby script for now.