Add module for Scheduler.

Also reorganizes some code to make it easier to add more modules.

test/scheduler/add.wren

@@ -0,0 +1,15 @@
+import "scheduler" for Scheduler
+import "timer" for Timer
+
+var run = false
+Scheduler.add {
+  run = true
+}
+
+// Does not run immediately.
+IO.print(run) // expect: false
+
+Timer.sleep(0)
+
+// Runs when the main fiber suspends on IO.
+IO.print(run) // expect: true