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/yield.wren

@@ -6,9 +6,9 @@ var fiber = Fiber.new {
   IO.print("fiber 3")
 }
 
-var result = fiber.call() // expect: fiber 1
+fiber.call()          // expect: fiber 1
 IO.print("main 1")    // expect: main 1
-result = fiber.call() // expect: fiber 2
+fiber.call()          // expect: fiber 2
 IO.print("main 2")    // expect: main 2
-result = fiber.call() // expect: fiber 3
+fiber.call()          // expect: fiber 3
 IO.print("main 3")    // expect: main 3