What other markdown viewers or editors support URL schemes that just execute code? And not in a browser sandbox but in the same security context notepad itself is running in.

Funnily enough, the core Windows API here that brings with it support for every URL scheme under the sun is plain old ShellExecute() from the mid-90s IE-in-the-shell era when such support was thought reasonable. (I actually still think it’s reasonable, just not with the OS architectures we have now or had then.)