Add library public interface

What is the new feature

Add a public interface for libfortran-support. This is related to #9

How is it implemented

Add a fortran_support.F90 collecting all subroutines and functions in the libfortran-support library.
Now one can do USE fortran_support to access all public subroutines/functions in the library.

cmake/gtest_helper.cmake

@@ -24,7 +24,7 @@ macro(add_icon_c_test test_name file_name)
   add_executable("CTest_${test_name}" ${file_name})
   target_link_libraries(
     "CTest_${test_name}" PRIVATE fortran-support::fortran-support
-                                 GTest::gtest_main stdc++fs)
+                                 GTest::gtest_main)
   add_test(NAME "CTest_${test_name}" COMMAND "CTest_${test_name}")
   set_property(TEST "CTest_${test_name}" PROPERTY LABELS C)
   set_target_properties("CTest_${test_name}"