I have moved the project into root environment. Like this the git integration in VSCode works as well.
Also the READMEs in the directories lib and test are gone.
The include directory has been removed successfull without any warning with redirecting the PIO include path
to be the src path.
Finally I removed the file extensions.json, because it is generated on each start of PlatformIO anyway.
(I had issues on my machine with line endings in this file anyway 😆)
this solves #16