build: 更新版本号至 2.1.9

- 在 package.json 文件中将版本号从 2.1.8 修改为 2.1.9

package.json

@@ -1,6 +1,6 @@
 {
   "name": "hexo-theme-solitude",
-  "version": "2.1.8",
+  "version": "2.1.9",
   "description": "A beautiful, powerful, and efficient Hexo theme developed by everfu.",
   "main": "package.json",
   "scripts": {