feat: allow yielding main fiber and expose current fiber reference

- Change Fiber.yield() to exit interpreter when no parent fiber exists instead of raising runtime error
- Add Fiber.current primitive to return the currently executing fiber object
- Update interpreter loop to stop execution when PRIM_RUN_FIBER receives null value
- Modify test expectations to reflect new yielding behavior from main fiber

test/fiber/yield_with_value_from_main.wren

@@ -1 +1,3 @@
-Fiber.yield(1) // expect runtime error: No fiber to yield to.
+IO.print("before") // expect: before
+Fiber.yield(1)
+IO.print("not reached")