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/transfer.wren

@@ -0,0 +1,24 @@
+var a = Fiber.new {
+  IO.print("a")
+}
+
+var b = Fiber.new {
+  IO.print("b before")
+  a.transfer()
+  IO.print("b after")
+}
+
+var c = Fiber.new {
+  IO.print("c before")
+  b.transfer()
+  IO.print("c after")
+}
+
+IO.print("start") // expect: start
+
+c.transfer()
+// expect: c before
+// expect: b before
+// expect: a
+
+// Nothing else gets run since the interpreter stops after a completes.