Linux CI and basic dependency management

[DarkWanderer]

This PR contains GitHub workflows and CMake changes needed to start an "official" Linux branch in Orbiter repo. The codebase itself does not compile yet - there is a number of errors still to fix, most notably the unmaintainable CMakeLists mess around LDoc/LFS. However, it is enough to allow cooperative work on Linux in central location

There are 2 options to incorporate this PR:

Option 1: Merge to main

If this change is merged to main, GitHub actions will start building Linux branch for each PR. It will be inconvenient as it will flag failures for good PRs, but it will allow to iterate the quickest

Option 2: Merge to separate linux branch

This way, work on finalizing Linux build can be kept separate from main, at the cost of potential desync with main branch and associated manual labor required to keep linux branch up to date with main

[n7275]

I don't have a super strong opinion on this, but I think as long as someone "owns" the linux branch (in a project management sense) I think option 2 is fine, with the goal of getting to a minimal build able output, even if it doesn't yet have a graphics client.

[jarmonik]

Last I had linux on my computer was about 20 years ago. It's been by intention to try FreeBSD but haven't got enough inspiration to risk wrecking a working computer. So, I don't know much about linux development. If I am writing a code that I know is not linux compatible then it could be isolated via "#ifndef __LINUX" but then again DX9 stuff shouldn't be linux compatible and there's lot of it. If linux stuff is merged to "main" I can't hold off commits and PR just because build it fails on linux due to reasons I don't know why. So, it would be up to a linux specialist to resolve the issues later on.