fix: Refs and cmd buffers, add unit tests, v0.1.0

- Refs and command buffers used to share data with every single world, this is now mitigated by using a separate data set for every world

- Working unit tests have been added

- jecs-utils has now been released at mark-marks/jecs-utils@v0.1.0!

lib/world.luau

@@ -4,15 +4,26 @@ local jecs = require("./jecs")
 
 local WORLD: jecs.World
 
+local listeners: { (jecs.World) -> () } = {}
+
 local function get(): jecs.World
     return WORLD
 end
 
 local function set(world: jecs.World)
     WORLD = world
+
+    for _, fn in listeners do
+        fn(world)
+    end
+end
+
+local function on_set(fn: (jecs.World) -> ())
+    table.insert(listeners, fn)
 end
 
 return {
     get = get,
     set = set,
+    on_set = on_set,
 }