Refactor fiber creation from a static `Fiber.create` method to a constructor-based `new Fiber` pattern. This introduces a `fiber_instantiate` native that returns the Fiber class itself, allowing `new Fiber` to invoke `fiber_new` for validation and creation. A shared `validateFn` helper is added to centralize function argument checking, replacing inline validation in both `fiber_new` and `fn_new`. All test files are updated to use the new syntax, and the old `create_wrong_arg_type` test is replaced with `new_wrong_arg_type`.