feat: replace fiber run() with transfer() and suspend() for scheduler-friendly semantics

Remove the run() method from fibers and introduce transfer() for non-stack-based fiber switching, along with suspend() to pause the interpreter and return control to the host application. Update Timer.sleep() to use runNextScheduled_() instead of Fiber.yield(), and add Timer.schedule() for creating independently scheduled fibers. Refactor the core fiber runtime to support transfer semantics, including proper error handling for aborted fibers and self-transfer edge cases. Fix a missing quote in test.py's runtime error validation string.

test/core/fiber/call_done.wren

@@ -0,0 +1,6 @@
+var fiber = Fiber.new {
+  IO.print("call")
+}
+
+fiber.call() // expect: call
+fiber.call() // expect runtime error: Cannot call a finished fiber.