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_called_fibers.wren

@@ -0,0 +1,21 @@
+import "timer" for Timer
+
+var a = Fiber.new {
+  IO.print("a before")
+  Timer.sleep(10)
+  IO.print("a after")
+}
+
+var b = Fiber.new {
+  IO.print("b before")
+  a.call()
+  IO.print("b after")
+}
+
+// All fibers are suspended since they were directly called and not scheduled.
+IO.print("before")  // expect: before
+b.call()            // expect: b before
+                    // expect: a before
+                    // expect: a after
+                    // expect: b after
+IO.print("done")    // expect: done