Utilize PROJECT_IS_TOP_LEVEL Variable

[threeal]

This issue suggests utilizing the PROJECT_IS_TOP_LEVEL variable to determine if the current sample project is configured at the top level. This suggestion will replace the SUBPROJECT variable, which is currently used to determine if the project is configured as a subproject. However, utilizing PROJECT_IS_TOP_LEVEL requires a minimum of CMake version 3.21. It's a worthwhile tradeoff for shorter code in the root CMakeLists.txt.