Provide directory of library's header as the include directory (#220)

Avoid having consumers need to know or declare internal structure of project.

Facilitate use with `FetchContent_Declare` to get the repo and then simply use `target_include_directories( their_proj PRIVATE csv)`.

include/internal/CMakeLists.txt

@@ -25,3 +25,4 @@ target_sources(csv
 
 set_target_properties(csv PROPERTIES LINKER_LANGUAGE CXX)
 target_link_libraries(csv PRIVATE Threads::Threads)
+target_include_directories(csv INTERFACE ../)