Add Fiber.try().

test/fiber/try_without_error.wren

@@ -0,0 +1,8 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+IO.print("before")  // expect: before
+IO.print(fiber.try) // expect: fiber
+                    // expect: null
+IO.print("after")   // expect: after