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_transferred.wren

@@ -0,0 +1,12 @@
+var main = Fiber.current
+
+var fiber = Fiber.new {
+  IO.print("transferred")
+  IO.print(main.transfer())
+  IO.print("called")
+}
+
+fiber.transfer() // expect: transferred
+IO.print("main") // expect: main
+fiber.call()     // expect: null
+                 // expect: called