Build for Linux

[fuzzybinary]

Linux build support needs support in https://github.com/fuzzybinary/dart_shared_libray (probably after fuzzybinary/dart_shared_library#5 is implemented). Then we should look into replacing the Visual Studio project files with Godot standard scons scripts.

[fuzzybinary]

There's duplicate work here from #9, specifically around transitioning to scons over Visual Studio.

[fuzzybinary]

Linux builds work, though they use CMake over scons.