Allow easier inclusion in other cmake projects

[doubleday]

This is a bit awkward because I'm such a cmake noob but this PR adds fn as a interface library which allows it to be used directly via target_link_libraries in other projects (using CPM.cmake here)

CPMAddPackage(
    NAME fn
    GITHUB_REPOSITORY ast-al/rangeless
)

target_link_libraries(MyApp PRIVATE
    fn
)

[doubleday]

Not sure whether the library name should rather be rangeless. Just took the project name from the CMakeLists.txt here.