To ensure that the rendering system is being run in the main thread, I
added the concept of "Sync systems", so that when a system group is
created it's possible to specify whether it's possible to run it on a
separate thread or not.
By defining archetypes using SOAs (Zig has this data structure in `std`,
called std.MultiArrayList), the engine can iterate faster over commonly
defined entities avoiding cache misses since each component is aligned
with other components of the same entity.
