Add Mac and Linux support

[uvcat7]

Project needs to be converted to use CMake
All the Windows API calls need to be stripped out and replaced with platform-dependent code

• System.cpp WinAPI calls (once [Refactoring] Port ArrowVortex to SDL3 #219 is complete)
• Debug.cpp console initialization (potentially there's an SDL option for this too)
• Write menu bar APIs for each OS, since SDL doesn't support them yet. Base on Native Menu Bars for Windows and MacOS libsdl-org/SDL#13752 so it's easy to migrate when it is supported.
• New CI items and build processes for each OS
• Support ARM/M1 architectures
• Port OpenGL code to SDL since Mac has deprecated OpenGL (is this necessary?)

[convextriangle]

related-ish - prototyping Qt here; will try to see how much maintenance/time/other cost it adds to the project

[uvcat7]

Two things missing from this:
Correct string vs. wstring handling
Cross-platform rendering for the menu bars
My vote for the menu bars is to use each OS's native menu bar platform. If that is too much of a pain then we can implement a menu in the editor itself for the non-Windows OSes.

[convextriangle]

related-ish - prototyping Qt here; will try to see how much maintenance/time/other cost it adds to the project

sorry, not enough free time in the upcoming days to implement the prototype - but for now, Qt seems like a breaking change and a big dependency (things such as building it from source via vcpkg seem to blow up build times and may cause other hassles for contributors), probably would agree with uvcat's vote (for now)

[sukibaby]

Port OpenGL code to SDL since Mac has deprecated OpenGL (is this necessary?)

Probably not for the time being. Apple has deprecated stuff and then taken over a decade to announce they're finally killing it off. Hopefully that is the case with OpenGL