clang-tidy applied and CMake installation of the single header added

[Spixmaster]

The commit messages already describe it.

The first commit does minor fixes suggested by clang-tidy which includes changing the headers. There are still many recommendation left but would require too much change which the owner needs to decide.

The second commit adds an installation of the single header. The problem is that headers like <inja/environment.hpp> themselves cannot be included because headers are missing. The library only works when including <inja/inja.hpp> or the single header.

All tests pass.

The project structure is really odd in a way that it does not include dependencies as headers but assumes that they are resolved by headers that include them which breaks the source code when the original files like <inja/environment.hpp> are included directly. I tried to solve this but then you will end up with circular dependencies. This should really change.

I am the packager for the AUR and these commits are needed so I would be very greatful if this gets approved.

Grüße aus Deutschland nach Deutschland.