Add cmake build support

[icegood]

link against library in other end-user cmakes should be done via

find_package(cppcoro 1.0.0 CONFIG REQUIRED)

target_link_libraries(${PROJECT_NAME} PRIVATE
coroutines::cppcoro
)

[Garcia6l20]

Many of us already proposed CMake support, you should check forks to avoid duplication.
The most advanced as far as I know is this one: https://github.com/andreasbuhr/cppcoro/tree/add_cmake_support