This PR addresses a fatal compilation error encountered on modern macOS environments (specifically using Xcode 15+ and modern Clang) where the C++ Technical Specification (TS) header <experimental/coroutine> has been removed in favor of the standardized C++20 header.

The error during compilation

.../mahi-util/include/Mahi/Util/Coroutine.hpp:24:6: error: "No coroutine header found. Ensure you are using a C++20 compiler."
    #error "No coroutine header found. Ensure you are using a C++20 compiler."
     ^
.../mahi-util/include/Mahi/Util/Coroutine.hpp:41:28: error: no member named 'experimental' in namespace 'std'
using SuspendAlways = std::experimental::suspend_always;
                      ~~~~~^

Changes:

Added a preprocessor check using __has_include to detect the presence of the standard header.

Implemented a conditional namespace alias to map std::coroutine_handle, std::suspend_always, and std::suspend_never into the std::experimental namespace.

This allows the existing codebase to remain unchanged while supporting modern C++20 compilers that have moved out of the "experimental" phase.

Maintains backward compatibility for older systems/Linux environments that still rely on the <experimental/coroutine> path.

Testing:

Verified on macOS (Sequoia/Sonoma) with CMake 4.x and Clang (Apple LLVM 15/16).
Confirmed that the fix resolves the 'experimental/coroutine' file not found error.
Notes: Requires the project to be compiled with the C++20 standard (e.g., -DCMAKE_CXX_STANDARD=20).

Steps done:

$> mkdir build && cd build
$> cmake .. -DCMAKE_CXX_STANDARD=20 -DCMAKE_POLICY_VERSION_MINIMUM=3.5
$> make

successfully compiled