#101 added memory leak check with Valgrind

[fktn-k]

Memory leak check with Valgrind has been added to this project to GitHub Actions workflows.
On local machines with Valgrind installed, the following commands execute the same check.

$ cd path/to/fkYAML
$ mkdir build && cd build
$ cmake .. -DCMAKE_BUILD_TYPE=Debug -DFK_YAML_BUILD_TEST=ON -DFK_YAML_RUN_VALGRIND=ON
$ cmake --build . --config Debug
$ ctest -C Debug -T memcheck

#101