feat: add toast.warn() fn

Author: Jostein Kringlen

src/core/toast.ts

@@ -48,6 +48,7 @@ toast.error = createHandler('error');
 toast.success = createHandler('success');
 toast.loading = createHandler('loading');
 toast.custom = createHandler('custom');
+toast.warn = createHandler('warn');
 
 toast.dismiss = (toastId?: string) => {
   dispatch({