feat: add Fiber.try() method for catching errors across fiber boundaries

Implement a new `Fiber.try()` method that allows a calling fiber to catch runtime errors from a called fiber. When a fiber is invoked with `try()` instead of `call()`, any runtime error that occurs in the called fiber is returned as a string to the caller rather than terminating the program with a stack trace. The `callerIsTrying` flag on `ObjFiber` tracks this state, and the `runtimeError` function now checks this flag to redirect the error to the caller's stack. Also adds `Fiber.error` getter to retrieve the error string from a failed fiber, changes `Fiber.isDone` to return `true` for errored fibers, and migrates the fiber error field from a raw `char*` to an `ObjString*` for proper memory management. Includes comprehensive test cases for try behavior, re-entry prevention, and error state queries.

test/fiber/is_done_after_error.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  "s".unknown
+}
+
+fiber.try
+IO.print(fiber.isDone) // expect: true