Consider refactoring include directory structure to match libzim

[ThisIsFineTM]

https://github.com/openzim/libzim/tree/main/include

libzim uses the include structure libzim/include/zim. To make zim-tools more consistent with libzim, I propose refactoring the current include structure to zim-tools/include/zim-tools.

If this is approved I can put in a PR.