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/timer/sleep_float.wren

@@ -1,15 +1,15 @@
 import "timer" for Timer
 
 // These are both rounded to 1, so "a" should complete first.
-Fiber.new {
+Timer.schedule {
   Timer.sleep(1.5)
   IO.print("a")
-}.call()
+}
 
-Fiber.new {
+Timer.schedule {
   Timer.sleep(1.3)
   IO.print("b")
-}.call()
+}
 
 IO.print("main")
 Timer.sleep(3)