fix: dpp::cluster::register_command with coroutine handler on windows

[usyless]

register_command with dpp::task was not working for me on windows with msvc, this fixes that, and it makes sure to keep register_command working for non coroutines.

I have not tested the changes on anything other than windows, therefore I'm unsure if im sacraficing other platforms.

Also I'm not sure if this is the ideal fix, however it does also let clangd correctly determine the function in use when ctrl+click'ing the register_command, as previously it would just use the non-coro one for all usages of register_command for me. (It is inspired by how dpp::cluster::start_timer works, which works correctly for me)

As of the latest commit it now only allows void or dpp::task function return signatures as well.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[Mishura4]

Tested on WSL g++ 14, clang++ 21, confirmed to work, merging after CI!

[Mishura4]

O, CI is already finished