Usage of API Guardian?

[laeubi]

Currently there is only API or non API in m2e... but that's quite weak. Sometimes a method or interface has to be public for technical reasons.

The API guardian provides as way to annotate things more clear:

https://github.com/apiguardian-team/apiguardian
https://apiguardian-team.github.io/apiguardian/docs/current/api/org/apiguardian/api/API.html
https://apiguardian-team.github.io/apiguardian/docs/current/api/org/apiguardian/api/API.Status.html

I have already seen this in another project and found it quite expressive and useful. Give that we might change a lot and even add new things with the 2.0 release, it might be good to mar things according to their current status.