Implement task sleep function

crates/lb/src/task.lua

@@ -1,6 +1,10 @@
+-- include task functions in the global scope
 local task = require("lb:task")
 
--- include `lb_tasklib::spawn` in the global scope
+function sleep(ms)
+  return task:sleep(ms)
+end
+
 function spawn(f, ...)
   return task:spawn(f, ...)
 end