feat: add scheduler module and refactor built-in module loading into shared modules.c

Introduce a new scheduler module with fiber-based cooperative multitasking, including native bindings for resume operations and a Wren class for scheduling callables. Refactor the existing timer module and CLI code to use a centralized built-in module registry in modules.c, replacing direct includes of timer.h and timer.wren.inc. Update Xcode project and Makefile to compile the new scheduler.c, modules.c, and their generated .wren.inc sources.

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