🎉 Added cross-platform CI

[AnotherFoxGuy]

Updated GitHub actions to run builds on Windows, Linux and MacOS
This uses Conan for installing the dependencies since it is the easiest way to get the cross-platform builds working
I will open a separate PR for generating artefacts that you can download and run for easy testing

[afritz1]

Anything that improves our use of GitHub Actions is a good thing. First time hearing about Conan.

Just glanced through the changes. I think CMAKE_BUILD_TYPE should be ReleaseGeneric because I'm not using the default build types.

Can look more tonight.

[afritz1]

I'd prefer avoiding changes to FindSDL2.cmake because I thought it's kind of a standard file. I don't understand the other CMake changes yet.

[Thunderforge]

To be clear: the usage will still keep us within the free tier of GitHub Actions, right? Given that there isn't any budget for this project, I think it would be rejected if it took us to the paid tier.

[AnotherFoxGuy]

The usage will still keep us within the free tier of GitHub Actions, right?

Yes, it absolutely should
From the gh-actions docs: GitHub Actions usage is free for standard GitHub-hosted runners in public repositories and these builds should be optimized enough to run on the GitHub-hosted runners