Added install target and cmake package exports

[gracicot]

This patch add the necessary cmake commands to export the package and config files.

This make inja easily consumable using plain cmake.

mkdir build && cd build
cmake ..

Then in consumer projects:

find_package(inja REQUIRED)
target_link_libraries(my_target PUBLIC pantor::inja)

I added inja targets to the pantor namespace. Tell me if you prefer the namespace to simply be inja.

Also, I added an option in the cmake to use the nlohmann json library found on the system. Unless specified, it will simply use the provided header.

The patch also make inja available through installation:

# inside inja/build
cmake ..
make install # installs the library in the installation prefix

[pantor]

Perfect, that’s awesome! Great to see contributions like this.