Add native cmake support

[LecrisUT]

This provides helpful cmake interfaces:

• Fypp_target_sources equivalent to target_sources. This is the main interface that should be used
• Fypp_add_library = add_library + Fypp_target_sources
• Fypp_add_executable = add_executable + Fypp_target_sources

The project is structured such that users can include fypp via FetchContent, e.g.:

cmake_minimum_required(VERSION 3.11)
project(test_fypp LANGUAGES Fortran)

FetchContent_Declare(
  Fypp
  # Pointing to my fork until it is merged
  GIT_REPOSITORY https://github.com/LecrisUT/fypp
  GIT_TAG cmake/cmake
)
FetchContent_MakeAvailable(Fypp)

Fypp_add_executable(test_fypp main.fypp)

Depends on #27, #29

[LecrisUT]

Going to wait for #27 and #29 because it is better to handle the call with python -m fypp